Frozen Bubble is a popular free puzzle game (especially in Linux platform). User controls a small canon, shooting bubbles and tries to hit other bubbles so to create groups of 3 or more bubbles of the same color. Whenever this is accomplished these bubbles go away. The purpose of the game is to eliminate all bubbles from stage.
I first played this years ago when I found it bundled with Mandrake linux. It's great to see this game maintained for Mac as well, and still free. A simple concept but very entertaining and playable for hours on end. Definitely worth a try.