Hi Giovi,
America's Army is well worth the effort to download. It's over 2Gb in size, but he game play is great. You do have to do your basic training before you can play online and until you've reached a score of 15 you will always start the game with insurgents weapons as opposed to the decent assault rifles. Though it doesn't take you long to get up to 15. The basic training consists of several excercises which unlock specific areas of the game. for example if you do the first aid part you can be a medic which is a useful attribute because you can score extra points for healing someone. There's lots more.
I've not played it for some time, but loved the game when I did play it with some friends. We even hired an America's Army server for several month and played it virtually every day.
You'll be seeing several of the games mentioned in the wiki freeware list over the next few months. It's been one of my sources for the free games for the weekday project.
Trackmania Nations is graphically superb fo free, but the singleplayer campaign is virtually non existent. I got two discs with trackmania on when I bought a couple of my graphics cards. If you like designing your own tracks then that is the game for you. :)
As for your question, I couldn't answer that because there are too many. Also it depends upon what mood I'm in at the time as to what I think is good.