As a 100% Linux user, most of my gaming these days is done on Freeciv, Battle for Wesnoth, or Scorched3d. If the battery on my Nokia N800 lasted longer, then I would consider using it for more gaming, but it's rather annoying to have your battery run down in the middle of trying to reset a wireless router. Hopefully, the Pandora should alleviate that concern.
For flexibility and modding though, there's nothing in the world that compares to a regular PC. I used to love the Wing Commander games, Freespace 2, and to some extent Freelancer. I've run through all of the Doom and Quake games, although the original Unreal still carries a warm place in my heart. For those times where you wish to return to nostalgia, you can fire up an emulator and return to the days of old. Quite frankly, some of those simple games are still much more enjoyable than the long, dragged out games of today.
I don't have as much time these days to play with groups (married with children versus being single), and most of my friends are scattered across the country now, but I enjoyed quite a few LAN parties in my days, and there's a real sense of community and belonging when guys can blow each other's characters apart and then go for a drink afterwards. The Wii is the closest thing that I've seen to that nowadays, and although some of the games are fun, nothing quite compares to packing up your computer and spending a whole night relaxing with people of common interests.