Puzzle Pirates
One of the more popular free games on the Net, Puzzle Pirates is a persistent MMO with a very casual bent. You create a cute pirate that looks like a Playmobil figure and sail the seas in search of pieces of eight. The catch is, each activity (and there are many, like manning the guns, repairing the ship, and bailing out bilgewater) is performed with a simple puzzle game.

Freeciv
It's Civilization, and it's free! Okay, so maybe this free and open-source strategy game "inspired by" the beloved Civ doesn't have all the nice UI features or pretty graphics of Civilization IV. But once you get the hang of the keyboard commands, it's really addictive.

Chalk
Joakim Sandberg's Chalk is good for a couple hours of free fun. The premise is simple—you guide your character around a chalkboard by right-clicking, and draw chalk lines with a left-click. Use your chalk lines to deflect bullets from enemies, remove obstacles, etc. The catchy tunes and sound effects help bring it all together into a nice, polished, scrolling "draw-er" of sorts.

F.E.A.R. Combat
As you get ready for the PC release of F.E.A.R. 2: Project Origin, why not brush up on your skills with the free multiplayer component of the original F.E.A.R.? The graphics hold up great—F.E.A.R. Combat is one of the best-looking free games on the Net.