PC game question (looking for games)

Hey all! I just joined the PC master race! Well, kind of. I bought a cheap laptop, I'm a cheap person. What are some great free/cheap games I can download (besides your ususal console roms) to fill up spare time?

EDIT: These are all great guys! A lot of these games I knew of, but needed my memory jogged to know what to start getting. Most excited about Grim Fandango and Half Life (I've played part two, never the first one). Oh yeah... and minecraft.