There are plenty of games you can run on anything, heck, back at my old work, I had a couple of games installed on my comp (yeah, not much work, those were the days) and also my 2004 laptop can run Counter-Strike Source with no problems at all. Just search a bit and you will find something that will suit your tastes, just search a bit on the Steam store or in GOG. As for my personal PC favorites, I could give you a couple of games that you may or may not like.
1. Wolfentein: Enemy Territory - Think of Team Fortress, but in WW2 and without silly hats. This game uses the Quake 3 engine, so yes, while it's a bit dated (and it can run on pretty much anything), it's still a lot of fun to play and the game is still very much alive, especially because it's absolutely, 100% free, no, it's not like those crappy corean games, this is the entire, full game you're getting, plus the immense ammount of user created content that's surprisingly great and even improves the core game itself, like new maps, weapons, modes, sounds... almost anything. Honestly, this is one of my all time favorite games, so I may be a bit biased towards it, but still, check it out, it's like a ~256 mb download and it's free, so, you've got nothing to lose.
2. Quake Live - Speaking of Quake 3, this is Quake 3, but for free and you can play it on any web browser on any computer. Quake 3 is also one of my favorite games and Quake Live is pretty much identical to Quake 3, but easier to get into, without having to download hundreds of mods.
3. Pretty much any Valve game, I can't decide wich one is my favorite, but Counter-Strike definitively isn't one of them. Check out Team Fortress 2, Day of Defeat Source and if you haven't already, Left 4 Dead 2. As for single player games, well, since you already said you have the Orange Box, there's not many games I can recommend you, unless you want to play Half-Life 2 with better graphics in HD resolution (HL2 of console version of the Orange Box is a port of the graphically inferior Xbox 1 version, but it's pretty much identical).
As for the games I had installed back at my old job, I had Unreal, Unreal Tournament and Plants vs Zombies.
You can't go wrong with those and if that peice of junk could run those, they will run in anything.