Mega Games Pack (MGP) by GlennLChugg is a standalone client for Ubuntu that allows you to download and play hundreds of free Linux games. Though there is Ubuntu Software Center (USC) and Desura client, MGP have some other useful features.
All games are free to install and play. MGP can launch any installed game from client itself.
Automatically adds your locally installed games to the client by searching .desktop files.
Uses GetDeb and other online repositories to update games.
Automatically fetches description and screenshots. You can also edit screenshots and description to add your own versions.
All games are neatly categorized into meaningful categories.
MGP is currently in beta testing mode so there will be bugs and issues. Also only one developer is working on it at the moment so keep that in mind. Some of the planned features include a themeable interface and support for global hotkeys.