Of all the freeware games I've ever played, this one takes the pie. TrackMania Nations Forever is an online racing game boasting gorgeous graphics, awesome physics, and a multiplayer mechanic that is like none other.
Gameplay wise, TrackMania Nations Forever is straight forward. Complete the track as quickly as possible. What I like about this game, however, is the abundance of linear tracks, rather than ordinary lap-based circuits. The levels are also a bit extreme, often with ramps and obstacles. Levels take a bit of work to master, but if you practice enough in solo mode, you'll definitely be prepared for online play.
Which brings me to my next point: multiplayer. Rather than using the usual race-based multiplayer system, TrackMania Nations Forever has implemented a "free-for-all" type of time-attack. Basically a level starts with a time limit (usually 5 minutes) and you're free to use this to complete the track in your best time. You can see the other players on the server, but there's no collision between you and the other drivers. Most tracks are roughly 20-30 seconds long, which means you can get in several full attempts easily and even more if you use the restart button. It sounds odd at first, but it's extremely well done.
Perhaps the coolest feature of this game is the instant restart button. And for once, instant MEANS instant. There are no loading screens, no nags, the game doesn't even hiccup. Trust me, the amount of times you'll need it makes this feature imperative.
Graphics wise, I've never seen a freeware game look so good. The screenshots don't even do the game justice. You have to play the game to see all the detail within.
Bottom line: Even if you hate racing games, you'll love TrackMania Nations Forever. So download it now or you'll be missing out on perhaps one of the best freeware games ever made.