Some of the games, if not all, can be install using djl - Game Manager, which is a game manager written in Python 2.5 for the GNU/Linux Operating Systems. It's says that it is inspired by Valve's Steam software for Windows.
It can also execute .desktop shortcuts located in another directory (this way, it's possible to launch games which were installed before djl).
Several games (see "List of games on repository" page) are available in the repository. Anyone can submit new games to developers via a web page. The list of games is regularly updated from the internet, so it's not static.
It can be install by adding the following server in the repositories (from terminal by editing the repositories list or by Software Sources from Ubuntu desktop: Go to System-Administration-Software Sources, Third-Party Software tab and add:):
deb http://archive.getdeb.net/ubuntu karmic-getdeb games
After that add the repository GPG key:
wget -q -O- http://archive.getdeb.net/getdeb-archive.key | sudo apt-key add -
After this you can go to http://www.playdeb.net/updates/Ubuntu/all#how_to_install to see a complete list of the games or http://www.jeuxlinux.fr (this site being in french you can use google translater to translate it in english: link).
You can install these games by clicking install this now underneath each game.