Need a completely free game? (And delightfully, free as in speech, as well as in beer, published under the GPL). How about Paintball 2.0?
Using a modified Quake 2 engine (but not needing Quake 2), what it exchanges in graphics it makes up for in speed. And although simple, there's some really nice level design here. As you'll have guessed, it's a multiplayer paintball game. But unlike real-life paintball, it doesn't SUCK OUT LOUD. Wow, I hate paintball. What a stupid, stupid way to spend your time. It bloody hurts, and the only way it can possibly be fun is if you get to vet every single person taking part for being not a complete shithead. But that won't happen because whenever you play there will be a group of neanderthals from a stag do, who think it's just so bloody funny to ruin everything by playing like morons, ignoring all the rules, and shouting their oh-so witty nicknames for each other before belly laughing at them for the 945th, then announcing just quite how much beer they're going to drink that night.
However, this doesn't appear to happen when playing Paintball 2.0. You can download the Windows and Linux clients from here, which are only 20.6Mb and 23.3Mb (1.44 and 1.63 Peggles respectively). You'll then need to set up an account (launched from within the game), and then you're in.
A quick tip: to make jumps that seem impossible at first, leap while still running up the slope before the obstacle, and you'll get a boost.