I notice that some games I bought at one point asked me for my email address (to register an account of sorts), and from then on, every month, or everytime there are new games out by them, they send out mails to me.

Ostensibly, it seems to be quite an effective way to keep your users in touch. But I suppose this would only work if you have a valid excuse for getting email address from the users (e.g for account setups).

I was thinking of using incentives (such as bonus functionality in-game) to beg for user's emails, but after that, what is the easiest way to keep track of their email addresses, and to send them a mail? What software can do that for you easily?

Also, is there web services for this? Not sure how much I'd trust web services not to harvest the mails instead though.