It would really help if you listed genres or games you enjoyed in the past.
Frozen Synapse
Great turn based strategy game. It easily runs on my rather low end netbook. You should have no issues with it. I highly recommended it.
DEFCON
Inspired by the ending of movie "Wargames", this RTS has you in control of one of the world's superpowers on the dawn of a nuclear war. A small, yet loyal following keeps this game going. Lots of mods to try out and the AI is pretty good.
World of Goo
A quirky puzzle builder, World of Goo has a great mixture of a unique art style, great gameplay, and a quirky sense of humor. You've got to at least try the demo.
Trackmania Nations Forever
This game reminds me a great deal of the old slot car sets I used to see when I was little. It isn't uncommon to see loops, jumps, and vertical banked turns in this free to play arcade racer. While there is a paid version, you'll be able to enjoy most of the online content with the free to play version.
Oddworld Series
Oh, where is there to say about this quirky platformer? While quite difficult, this quirky game series sends you on an amazing journey. While I'm not sure if you'll be able to run the last two games in the series, I do know that the first two will be easily handled by your computer.
Spacechem
Perhaps my favorite puzzle game. Spacechem looks far more complex than it really is. Using a combination of chemistry and basic programming, this puzzle game will have you searching for the best and most efficient solutions. With several ways to solve each of the game's challenges, you can spend many hours refining your solutions to be as efficient or quick as possible. Download the demo and try it.
X Series
The X series reminds me a great deal of the late 90s game Freelancer. Only in much grander scale, without the repeating dialog, and better written. You'll most likely be able to run the first couple games of the series, and maybe the third with lowered graphics, it is a great space sim where you can go do what you want. Want to go reek havoc? Get a decent space ship and start blowing up stations! Want to be a trader? So be it. It is a great space sandbox. Be warned, the later games do get quite complex. You may need to watch some videos to get up to speed.
Revenge of the Titans
The first of two tower defense games, Revenge of the Titans is a bit different from the norm. For one, there is no set lanes. So, creeps can break off from the pack and completely bypass your defenses. Also, there is a great assortment of weapons and upgrades to buy and research. A scaling difficulty will make it so that you're always facing a challenge, no matter how much money you're able to put away.
Defense Grid: The Awakening
Of course, the big name in tower defense has to be Defense Grid. This is not without reason. A great story, strong voice acting, and challenging levels makes for a great overall game.
