World of Padman
With so many open source first person shooter games available online, it's often tough to choose which title to give your precious hard drive space to. Of course, you might have several criteria in mind for deciding, like gameplay or graphics. One game that would pass both categories with flying colors is World of Padman.
World of Padman has a similar gameplay with Quake III Arena, minus CTF mode. The graphics are quite cool and colorful while the gameplay is definitely perfect for trigger happy gunslingers.