I know Ubuntu comes with a "Remote Desktop" option that appears to be a straightforward VNC server, and I'm trying to understand the alternatives. Here are the possibilities I've heard about so far:

• VNC
• VNC + SSH Tunnelling
• NX Server, free edition
• FreeNX
• NeatX
• X2Go
• X11 Forwarding over SSH
• xrdp

I'm coming at this from a Windows user's perspective: To the best of my experience, RDP (aka Terminal Services) is a reasonably secure (barring mitm/server spoofing), efficient desktop sharing protocol with well-supported clients, that can be exposed to the internet when necessary without major fears of intrusion. To the best of my knowledge straight VNC is none of those things, which is where I get confused - why wouldn't a better desktop sharing technology be developed or used in the open-source world? I know VNC can be wrapped with SSH, but that seems beyond the reach of a casual user. X11 forwarding over SSH may be more or less efficient, I have no idea, but is definitely even more complicated, and doesn't (as far as I know) give you access to already-running stuff (no desktop sharing as such, just remote application running).

So, I'd like any feedback/preferences amongst these or any other "Free" desktop sharing options, using these criteria and/or any others:

1. Security (esp. for access across internet)
2. Efficiency (bandwidth usage, responsiveness, etc)
3. Free-ness, as in Speech (not sure where RDP or FreeNX lie for this)
4. Free-ness, as in Beer (are there any commercial solutions with usable dependable free offerings?)
5. Ease of use (server and client side)
6. Cross-OS Client availability
7. Cross-OS Server availability
8. Support for independent sessions and shared (and/or "Console") sessions
9. Ongoing support/maintenance/development

Thanks!