TrackMania Nations Forever
Or, more commonly for me, CRASH!
TrackMania Nations Forever is one of the slickest and most polished freeware games I have seen, and my utter incompetence behind the wheel ... or keyboard ... is no slight on the game itself.
TrackMania is a racing game with an extremely simple interface. For almost all purposes, you need just four keys: the arrows. Forward, left, right, brake. Pretty darn simple. The complexity and beauty of TrackMania comes in the tracks--there are 65 included in the game, of varying complexity. Many are insane rollercoaster tracks which could never exist in real life, full of twisting loops, massive gaps which must be crossed precisely at the right angle, and even regions which will accelerate or slow your car. I, personally, spent a lot of time observing the well-done physics engine, meaning, I got to watch my car careen over obstacles, plummet through holes, and bounce majestically across the terrain. There are no explosions, fire, or shrapnel, however.
In addition to being able to crash over and over in private, TrackMania Nations Forever has an online component, where you can join hundreds of thousands of other players around the globe and discover just how badly you play, as their cars zoom by you with grace and speed while you (or, to be fair, me), can't seem to make it past the first twisting spiral out of the gate.
Add in a track editor and a car customizer (change colors, add decals, and so on), and the value TrackMania Nations Forever gives you for your payment of no dollars and no cents is phenomenal.
Note: This link takes you to the vendor's site, where you can download the latest version of the software.