Linux

You've probably heard of Nethack (wikipedia entry). It is a rogue-like adventure game that has been under continuous, if slow, development since the late 1980s. It has great gameplay and more functions and surprises than most commercial "hack and slash" RPGs. Unfortunately, it has extremely primitive "graphics" -- letters, numbers, and other characters on a text screen just like in the 1980s when DOS was the operating system of choice.

Google leaves programs in "beta" long after most companies would have released a "release" version. In the case of the Google Earth beta, Google is just released its 4th major release and the program is still considered "beta." In case you are one of the rare people who has never heard of Google Earth, this program allows you to view Earth from space and then get very close to just about anywhere on the planet via satellite images. It's a lot of fun to play with and even has real world uses.