Well, I game on my laptop and I can run most modern games fine.
There's a wide variety of laptop specs out there - some of the newer ones outperforming consumer-grade desktops. (Edit) Meaning: you might be better served if you post your laptop model/specs and we may be able to tell you what may/may not run smoothly on your system.
But, assuming you can run Half Life 2 fine, I'd recommend some of the more "Indie" titles or older ones as there are plenty of great games out there - or even other games running the source game engine (same engine running Half Life 2).
- If you're looking for single player, Portal is very, very good; and a sequel is coming out soon.
- Multiplayer you can do no wrong with Team Fortress 2 and Counter-Strike: Source.
- You can probably play this one one your laptop: and some may say that this is a terrible game, but I did rather enjoy myself playing Doom 3 and Return to Castle Wolfenstein (not the Wolfenstein remake that came out on 2009, didn't play that one)
- Torchlight is a lot of fun, doesn't have very high requirements
- Revenge of the Titans is a fun game: top down, stylized 2D graphics
So on and on.