Remember the old cellphone game Snake? It's been around longer than I've been alive, but became a hit once it became standard on Nokia cellphones. I have fond memories of standing in cold registration lines in college and playing the game as I waited to sign up for classes.
Screen Snake is a free version of that game that's been ported to the Mac. It's as easy to play as the cellphone version and launching the game automatically starts the snake crawling across your Mac's screen. There's a few options for changing the snake size, but there's no ability to pause the game at this time or keeping score on the free version. The paid version (US$1.99) adds more levels and scores.
It's a good time-waster, and if you're looking for a splash of nostalgia, this is a good game to keep around.