If you find yourself with time on your hands and nothing to do on this dull Thursday afternoon, allow me to direct your attention to Altitude. It's a cute, cartoony little 2D multiplayer aerial combat game with multiple modes of gameplay and an unexpectedly sophisticated control scheme. More to the point, developer Nimbly Games is now offering it completely free.
I've only spent a few minutes with Altitude so I can't offer any definitive opinions, but PC Gamer UK said it's "deceptively deep [and] rewarding to master" and I hear they're a pretty solid bunch of guys. Plus it's a relatively tiny download—62 meg or thereabouts—so why not?
And in fact, it does look pretty good. It offers multiple free-for-all and team-based gameplay modes, Call of Duty-style perks, a variety of power-ups, and a throttle-based flight model complete with stalls. And did I mention it's free? Because it is.