macosxhints.com has published a review of DOSBox for Mac OS X. DOSBox is a DOS environment that runs under Mac OS X (as well as several other OSes), and is primarily used to play old DOS-based computer games.
Macosxhints.com listed it as their Pick of the Week for August 28, and the reviewer mentions what most people use it for: DOSBox isn’t rich on user interface features, but it works quite well. There’s even SoundBlaster support, so you get in-game sound. I’m sure DOSBox could be used for more productive tasks as well, but it’s great as a flashback gaming machine!