Good free games don't come along very often. Zaz is one of them.
According to the description of the author, Zaz "...is a puzzle game where the player has to arrange balls in triplets." Don't be fooled by the short description - Zaz has quite a few levels ranging from moderate to difficult - to go through.
Personally, I am struggling to get past level 7, and I don't feel like giving up - the game is good enough to keep me going. On the other hand, I know someone that finished the game in 4 days only. So maybe it's just me thinking it's a tough (but fun) game.
However, the overall experience it's great, the play is smooth, great sounds and special effects (balls exploding). So if you think you're up to the challenge and really fast with the mouse, go ahead and give it a try - you can grab it from Sourceforge. Zaz runs on Linux and Windows as well.