For Nintendo fans, or those who loved Super Mario, Secret Maryo Chronicles is a decent Mac adaption of the classic game - and it's completely free.
Secret Maryo Chronicles is an open source two-dimensional platform game in the side-scrolling style. The gameplay measures up reasonably well with the original and if you get bored at any stage, there's a nice bonus of a level editor to create your own game. By pressing F8, you access the level editor and a whole world of backgrounds, graphics and characters to create your own Maryo world.
The aim of the game is the same as the 1992 (has it really been that long!?) original - collect as many gold coins as possible while squashing and avoiding baddies along the way. Secret Maryo Chronicles features a series of extra baddies and challenges absent from the original that make it slightly harder, but if you collect 100 coins, you gain an extra life.
Super Mario fans who have always missed the chance to play the classic game on the Mac, will enjoy Secret Maryo Chronicles and love the level editor.