Doom - http://www.idsoftware.com/games/doom/doom-final - The original. Source code is available on id's website, or you can use one of the Doom ports listed below.
Boom - http://www.teamtnt.com/boompubl/boom2.htm - One of the first Doom ports released after id Software released the source code in 1997. No longer maintained. Most current Doom ports are based off of this port.
Doom Legacy - http://legacy.newdoom.com - A port of Doom for Linux and Windows, Doom Legacy adds a lot of cool new features, at the expense of compatibility with older release versions of Doom.
atari800 - http://atari800.sourceforge.net - 8-bit Atari Computer emulator for Linux. This emulator works really well, lets you do lots of crazy stuff with old 8-bit Atari software.
xpilot - http://www.xpilot.org - An Asteroids/Star Trek style game, a Unix staple for years. Good thing about xpilot is that it's massively multiplayer, and cross-platform as well, so you can play across the internet.
Quake - What more do you need to know? Quake was the next generation after Doom in id Software's arsenal. Hopefully by the time a Quake module is written, OpenGL support for PortaBoom will be ready for the world :) http://www.idsoftware.com/games/quake/quake3-gold
Samba - http://www.samba.org - Samba is a network filesharing utility for Unix machines that allows Unix machines to share files with Windows machines, and vice versa. Samba is used in PortaBoom to access files on Windows computers, and Unix NFS is used to access files on Unix machines.
LinuxCare Bootable Business Card - http://lbt.linuxcare.com/index.epl - A complete Linux toolkit on a business card sized CD. A lot of my PortaBoom ideas came from this project.
Debian GNU/Linux - http://www.debian.org - Debian is a free operating system (OS) for your computer. An operating system is the set of basic programs and utilities that make your computer run. Debian uses the Linux kernel (the core of an operating system), but most of the basic OS tools come from the GNU project; hence the name GNU/Linux.
ALSA - http://www.alsa-project.org - Advanced Linux Sound Architecture, an alternate sound card implementation for Linux. Here's a list of ALSA supported sound cards, along with the names of the module to load for a specific card.