GeneRally is a freeware portable racing game which squeezes a surprising amount of entertainment into its very minimal 4MB of hard drive space.
The program is very configurable, for instance. Launch GeneRally and you can choose from 36 tracks, as well as defining how many and which competitors you'd like to drive against. You can also create new drivers, customise what they're driving, even tweak key settings to alter how the game works ("tyre wear", "fuel consumption", "fuel mass" and more).
What you don't get here is much in the way of graphics, of course. The cars are tiny blobs of pixels, the tracks basic, so we'd recommend keeping your expectations low. (And even then you might be disappointed.)
Still, GeneRally is very playable. The cars are easy to control (just use the cursor keys), you can compete against the computer or other human players, and with plenty of extra cars and tracks to download online there's probably hours of fun to be had here.
The graphics are extremely basic, but GeneRally gets the gameplay right, and if you like this kind of arcade-style racing game then you should have a lot of fun here