Christopher Hyde writing for Critical Distance has done the leg work here - generating an extensive list of the 99 Free games which he deems to be noteworthy in 2009. After wading through a bunch of them that I hadn't tried before, I've got my picks so far below.

Not included in the list were a couple which I would put up there: Obechi - a pretty, quick-click game involving rings and dots; and Every Day the Same Dream - another Molle Industria specialty dealing with "alienation and the refusal of labour". Honorable mention also goes to Canabalt.

The picks:

Gretel and Hansel
Artistically beautiful, pleasing on the ear, and darkly humorous, I am looking forward to the next installment. Outside of a few navigational judder bars this would have to be one of my favourite casual games ever.

Plants vs. Zombies (Flash Version)
What can I say, I'm addicted to this game anyway so I couldn't leave it out. Having clocked it (various times) already, I still couldn't resist a wee dabble... just one more level... I swear. Thank god for limited demos is all I can say.

Effing Hail
This game I enjoyed more for its meaningful play element than anything else (however small), but it is still great fun to blow houses up with massive hail storms.

Use Boxmen
Great game despite my wanting to strangle it by level 7, the art is neato and the difficulty ramps like nobody's business. A very well executed puzzle game.

Cursor Chaos
Channelling WarioWare only with less introduction and more cohesive art style, this is a well executed gem of a game that will have you challenging your friends.