http://gbatemp.net/threads/the-50-best-freeware-games.290310/
^^^^^^^^^^
Really fucking awesome. Tons of low-spec free games.
Rules of Thumb:
Any Source game. Really smooth and well optimized engine. Not ALL will run smooth, but they will start up and at least be playable.
Any PC title form before 2007, seriously, they ALL run great on Intel 4000. But check the specs if your buying.
Some of my personal favorites.
Halo CE & 2, ran great on my schools Pentiums.
Noitu Love 1 and 2. On that list above, but I love it so much i'll put it here too.
Cataclysm DDA. Rouge-like, my main in fact. Me and some buddies set up an SSH and play a dedicated world together.
Dwarf Fortress. Great if you have the patience to learn it. If not, Adventure mode rampages are always fun!
Blockland. Fun little sandbox, I dont know how good it runs with shaders on Intel 4000 though.
I'll add to this post as I remember.