I am 80% done of my 4th game and I am going to release it as a freeware!

Any tips for using it as a boosting/promotion tool?

I presume your goal is to attract users' attention to other commercial games you will be selling. If so, then have a forum for the game and announcement mailing list. I have a free game that has been building a user base for years now. The forum has 600+ users and I'm averaging 25 new users each month. It's hard to say it without sounding Machiavellian, but man, I can't wait to sell games to all these people! (I'm just about there, but that's another story.)

If you've got a game that needs hints or has a level editor, it is a much better candidate for building a user community. Obviously anything that encourages communication between users is going to be more helpful. If you just have something like a Tetris game, then the forum would not be so valuable. Myself and other people have poured a lot of time into the forum, because it is fun to be connected to the people that play your game, but also it helps tremendously to keep users interested in what we will be releasing next. So we've hacked the forum software to allow showing rooms from the game (used when asking for help on the forum), uploading user-created level collections, and numerous other things. Every month we have a contest and give away game-related t-shirts, mugs, and posters. This effort gives us enthusiastic users that are really on our side.

-Erik