One of my favourite games in Linux is XMoto. It is so simple yet so addicting. It is a side scrolling motocross game; in fact it looks more like riding a trial motorcycle. The physics, although not completely realistic (eg: the angular momentum isn't conserved during the flight after a jump) , feel really good and smooth. The game is updated regularly and gives you the ability to download new levels, skins and world records (along with a replay of the record) without even closing the game. This way you can play straight for days if you want to even have a chance to break one of the world records. Soon you will start admiring (=hate) some names like: "Duuferna" or "Destroyer". The graphics are simple but adequate, some levels have their own textures too and a few of them look like other classic games like Sonic.
XMoto can be found in the Ubuntu Linux repositories so you can find and install it with Synaptic or by issuing the command apt-get install xmoto in the console. Windows users may visit the official site of XMoto and download the windows installer.