Three Free to Play games, all available on Steam.
All three working on GMA, check it out.

PUZZLE PIRATES

PUZZLE PIRATES is fun little MMO with puzzle games. It is simple graphical game so it works
on GMA 4500 without any problem.
It gets 40+ FPS and it is totally playable.

War Inc. Battlezone

War Inc. Battlezone is FPS and it uses MMO elements, match making, progress and many others.
It works in 9-20 FPS on all lowest settings.
But I didn't find it enjoyable to play on lowest settings and low resolution with ocassional frame
drops.
I thought about editing configuration files, but there are no config files.
If you like it, play it :) because it works.

Iron Grip: Marauders

This is nice RTS with MMO elements, it is browser game.
It runs in 18-28 FPS on medium settings and resolution, it is playable and fluid.

That's all for now, enjoy!