Urban Terror
If you liked
Quake III Arena, that's actually where I started playing shooters. It was a
Quake III Team Arena mod, and now it's a free game of its own. It's not a Steam title, but it's such a good game. It makes you wonder how that game has more-advanced features than modern shooters.
Also,
Rage is pretty good, it's just not great. I really thought the new id Tech 5 engine was cool in it, how you can shoot a running enemy and he'll stagger based on where he's shot. You could always look into the
Quake and
Unreal combo packs, if you want to play those games still. You haven't really explained what all you're looking to play now too well, so I don't have any specific suggestions.
That said, regardless of what you want to get, I'd wait a week or two before I got into buying things heavily. The Winter Sale will have 1,000+ games on-sale at crazy prices, and oyu can load up at that time. You should have enough with the Humble Bundle to keep you occupied.