Steam

Installation - Ubuntu Linux: How To
Steam can be installed via the Software Centre but it does require setting up an Ubuntu One account, if you do not want to use the services then just head over to Steam's website.

With any luck the website will recognise you are running Linux and display an 'Install Steam' button towards the top right hand side of the web page. If not just click on the 'Linux' section and that 'Install Steam' button should now be visible. Click through to the next page and download the .deb file.

Once downloaded you can either install the .deb file via the command line, see here, or if you and GDebi installed you can just right click on the installer and select 'Open with GDebi Package Installer'.