I found a little indie game on Steam a while back.  It's an incredibly simple, daft game.  You have a plane.  It speeds up, slows down, turns and shoots.  It uses flat side-on 2D graphics and some of the most simple physics possible.  It has simple multiplayer (throw players into an online arena with a set of objectives and let them get on with it).

And yet the damn thing has sucked up hundreds of hours of my life.

The game is Altitude.  You can play it for free in your browser, even joining the same servers as everyone else, but the full game is standalone and a lot more interesting.  And most importantly, it's dirt-cheap, and also available via Steam, so you don't have to worry about keeping the download file or remembering your account details, or writing down your registration keys, etc.

After a while, the only downside to its multiplayer aspect was revealed - the ubiquitous Online Idiot.  So I started my own server in order to escape them (or at least be able to get rid of them once they appeared) and play with a small group of people who were interested in the game more than their own personal score or how much trouble they could cause.

So now I have 6 Altitude servers.  You can find them in the game's server browser, they all start with the word "Ledow".