I have to different ISPs, each on each own network. The main connects via ethernet and the secondary via wifi. The two networks have no relation at all. I just connect to them simultaneously. The reason I want to load balance between them is to achieve higher Internet speeds. Note: I have no advanced network hardware. Just my pc and the two routers that I have no access... main network:

if: eth0
gw: 192.168.178.1
my ip: 192.168.178.95
speed: 400 kbit/s

secondary network:

if: wlan0
gw: 192.168.1.1
my ip: 192.168.1.95
speed: 300 kbit/s

A diagram to explain the situation: http://i.imgur.com/NZdsv.jpg

I'm on Arch Linux x64. I use netcfg to configure the interfaces Configs:

# /etc/network.d/main
CONNECTION='ethernet'
DESCRIPTION='A basic static ethernet connection using iproute'
INTERFACE='eth0'
IP='static'
ADDR='192.168.178.95'

# /etc/network.d/second
CONNECTION='wireless'
DESCRIPTION='A simple WEP encrypted wireless connection'
INTERFACE='wlan0'
SECURITY='wep'
ESSID='wifi_essid'
KEY='the_password'
IP="static"
ADDR='192.168.1.95'

And I use iptables to load balance, rules:

#!/bin/bash
/usr/sbin/ip route flush table ISP1 2>/dev/null
/usr/sbin/ip rule del fwmark 101 table ISP1 2>/dev/null
/usr/sbin/ip route add table ISP1 192.168.178.0/24 dev eth0 proto kernel  scope link  src 192.168.178.95  metric 202
/usr/sbin/ip route add table ISP1 default via 192.168.178.1 dev eth0
/usr/sbin/ip rule add fwmark 101 table ISP1
/usr/sbin/ip route flush table ISP2 2>/dev/null
/usr/sbin/ip rule del fwmark 102 table ISP2 2>/dev/null
/usr/sbin/ip route add table ISP2 192.168.1.0/24 dev wlan0 proto kernel  scope link  src 192.168.1.95  metric 202
/usr/sbin/ip route add table ISP2 default via 192.168.1.1 dev wlan0
/usr/sbin/ip rule add fwmark 102 table ISP2
/usr/sbin/iptables -t mangle -F
/usr/sbin/iptables -t mangle -X
/usr/sbin/iptables -t mangle -N MARK-gw1
/usr/sbin/iptables -t mangle -A MARK-gw1 -m comment --comment 'send via 192.168.178.1' -j MARK --set-mark 101
/usr/sbin/iptables -t mangle -A MARK-gw1 -j CONNMARK --save-mark
/usr/sbin/iptables -t mangle -A MARK-gw1 -j RETURN
/usr/sbin/iptables -t mangle -N MARK-gw2
/usr/sbin/iptables -t mangle -A MARK-gw2 -m comment --comment 'send via 192.168.1.1' -j MARK --set-mark 102
/usr/sbin/iptables -t mangle -A MARK-gw2 -j CONNMARK --save-mark
/usr/sbin/iptables -t mangle -A MARK-gw2 -j RETURN
/usr/sbin/iptables -t mangle -A PREROUTING -j CONNMARK --restore-mark
/usr/sbin/iptables -t mangle -A PREROUTING -m comment --comment "this stream is already marked; escape early" -m mark ! --mark 0 -j ACCEPT
/usr/sbin/iptables -t mangle -A PREROUTING -m comment --comment 'prevent asynchronous routing' -i eth0 -m conntrack --ctstate NEW -j MARK-gw1
/usr/sbin/iptables -t mangle -A PREROUTING -m comment --comment 'prevent asynchronous routing' -i wlan0 -m conntrack --ctstate NEW -j MARK-gw2
/usr/sbin/iptables -t mangle -N DEF_POL
/usr/sbin/iptables -t mangle -A DEF_POL -m comment --comment 'default balancing' -p tcp -m conntrack --ctstate ESTABLISHED,RELATED -j CONNMARK --restore-mark
/usr/sbin/iptables -t mangle -A DEF_POL -m comment --comment 'default balancing' -p udp -m conntrack --ctstate ESTABLISHED,RELATED -j CONNMARK --restore-mark
/usr/sbin/iptables -t mangle -A DEF_POL -m comment --comment 'balance gw1 tcp' -p tcp -m conntrack --ctstate NEW -m statistic --mode nth --every 2 --packet 0 -j MARK-gw1
/usr/sbin/iptables -t mangle -A DEF_POL -m comment --comment 'balance gw1 tcp' -p tcp -m conntrack --ctstate NEW -m statistic --mode nth --every 2 --packet 0 -j ACCEPT
/usr/sbin/iptables -t mangle -A DEF_POL -m comment --comment 'balance gw2 tcp' -p tcp -m conntrack --ctstate NEW -m statistic --mode nth --every 2 --packet 1 -j MARK-gw2
/usr/sbin/iptables -t mangle -A DEF_POL -m comment --comment 'balance gw2 tcp' -p tcp -m conntrack --ctstate NEW -m statistic --mode nth --every 2 --packet 1 -j ACCEPT
/usr/sbin/iptables -t mangle -A DEF_POL -m comment --comment 'balance gw1 udp' -p udp -m conntrack --ctstate NEW -m statistic --mode nth --every 2 --packet 0 -j MARK-gw1
/usr/sbin/iptables -t mangle -A DEF_POL -m comment --comment 'balance gw1 udp' -p udp -m conntrack --ctstate NEW -m statistic --mode nth --every 2 --packet 0 -j ACCEPT
/usr/sbin/iptables -t mangle -A DEF_POL -m comment --comment 'balance gw2 udp' -p udp -m conntrack --ctstate NEW -m statistic --mode nth --every 2 --packet 1 -j MARK-gw2
/usr/sbin/iptables -t mangle -A DEF_POL -m comment --comment 'balance gw2 udp' -p udp -m conntrack --ctstate NEW -m statistic --mode nth --every 2 --packet 1 -j ACCEPT
/usr/sbin/iptables -t mangle -A PREROUTING -j DEF_POL
/usr/sbin/iptables -t nat -A POSTROUTING -m comment --comment 'snat outbound eth0' -o eth0 -s 192.168.0.0/16 -m mark --mark 101 -j SNAT --to-source 192.168.178.95
/usr/sbin/iptables -t nat -A POSTROUTING -m comment --comment 'snat outbound wlan0' -o wlan0 -s 192.168.0.0/16 -m mark --mark 102 -j SNAT --to-source 192.168.1.95
/usr/sbin/ip route flush cache

(this script was made by fukawi2, I don't know how to use iptables) but I have no Internet connection...

output of iptables -t mangle -nvL

Chain PREROUTING (policy ACCEPT 1254K packets, 1519M bytes)
 pkts bytes target     prot opt in     out     source               destination         
1278K 1535M CONNMARK   all  --  *      *       0.0.0.0/0            0.0.0.0/0            CONNMARK restore
21532   15M ACCEPT     all  --  *      *       0.0.0.0/0            0.0.0.0/0            /* this stream is already marked; escape early */ mark match ! 0x0
  582 72579 MARK-gw1   all  --  eth0   *       0.0.0.0/0            0.0.0.0/0            /* prevent asynchronous routing */ ctstate NEW
 2376  696K MARK-gw2   all  --  wlan0  *       0.0.0.0/0            0.0.0.0/0            /* prevent asynchronous routing */ ctstate NEW
1257K 1520M DEF_POL    all  --  *      *       0.0.0.0/0            0.0.0.0/0           

Chain INPUT (policy ACCEPT 1276K packets, 1535M bytes)
 pkts bytes target     prot opt in     out     source               destination         

Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination         

Chain OUTPUT (policy ACCEPT 870K packets, 97M bytes)
 pkts bytes target     prot opt in     out     source               destination         

Chain POSTROUTING (policy ACCEPT 870K packets, 97M bytes)
 pkts bytes target     prot opt in     out     source               destination         

Chain DEF_POL (1 references)
 pkts bytes target     prot opt in     out     source               destination         
1236K 1517M CONNMARK   tcp  --  *      *       0.0.0.0/0            0.0.0.0/0            /* default balancing */ ctstate RELATED,ESTABLISHED CONNMARK restore
15163 2041K CONNMARK   udp  --  *      *       0.0.0.0/0            0.0.0.0/0            /* default balancing */ ctstate RELATED,ESTABLISHED CONNMARK restore
  555 33176 MARK-gw1   tcp  --  *      *       0.0.0.0/0            0.0.0.0/0            /* balance gw1 tcp */ ctstate NEW statistic mode nth every 2
  555 33176 ACCEPT     tcp  --  *      *       0.0.0.0/0            0.0.0.0/0            /* balance gw1 tcp */ ctstate NEW statistic mode nth every 2
  277 16516 MARK-gw2   tcp  --  *      *       0.0.0.0/0            0.0.0.0/0            /* balance gw2 tcp */ ctstate NEW statistic mode nth every 2 packet 1
  277 16516 ACCEPT     tcp  --  *      *       0.0.0.0/0            0.0.0.0/0            /* balance gw2 tcp */ ctstate NEW statistic mode nth every 2 packet 1
 1442  384K MARK-gw1   udp  --  *      *       0.0.0.0/0            0.0.0.0/0            /* balance gw1 udp */ ctstate NEW statistic mode nth every 2
 1442  384K ACCEPT     udp  --  *      *       0.0.0.0/0            0.0.0.0/0            /* balance gw1 udp */ ctstate NEW statistic mode nth every 2
  720  189K MARK-gw2   udp  --  *      *       0.0.0.0/0            0.0.0.0/0            /* balance gw2 udp */ ctstate NEW statistic mode nth every 2 packet 1
  720  189K ACCEPT     udp  --  *      *       0.0.0.0/0            0.0.0.0/0            /* balance gw2 udp */ ctstate NEW statistic mode nth every 2 packet 1

Chain MARK-gw1 (3 references)
 pkts bytes target     prot opt in     out     source               destination         
 2579  490K MARK       all  --  *      *       0.0.0.0/0            0.0.0.0/0            /* send via 192.168.178.1 */ MARK set 0x65
 2579  490K CONNMARK   all  --  *      *       0.0.0.0/0            0.0.0.0/0            CONNMARK save
 2579  490K RETURN     all  --  *      *       0.0.0.0/0            0.0.0.0/0           

Chain MARK-gw2 (3 references)
 pkts bytes target     prot opt in     out     source               destination         
 3373  901K MARK       all  --  *      *       0.0.0.0/0            0.0.0.0/0            /* send via 192.168.1.1 */ MARK set 0x66
 3373  901K CONNMARK   all  --  *      *       0.0.0.0/0            0.0.0.0/0            CONNMARK save
 3373  901K RETURN     all  --  *      *       0.0.0.0/0            0.0.0.0/0