When I was a kid, it seemed that Doom was a kind of legendary game that everyone had played at some stage or other.
By today's standards it looks pretty tame, but those who are nostalgic can roll back the years with the strangely named PrBoom. PrBoom is a an open source, cross-platform port clone of the original Doom. To get started, once you've downloaded PrBoom, you'll need to install the original Doom data files to be able to play. Depending on your Doom version these are called 'doom.wad', 'doom2.wad', 'tnt.wad' or 'plutonia.wad'.
You can download them from Doomworld, selecting the appropriate file. Finally, you'll need to place the files in '~/Library/Application Support/PrBoom' so that PrBoom can find them.
The great thing about PrDoom is that it's harnessed the processing power of Macs and taken Doom to another level, with much better controls and multi-player support. PrDoom also introduces features such as an uncapped framerate, variable gamespeed, re-record, walkcam, chasecam, hi-res, and detailed graphics, without losing the essence of the original Doom. It does, however, require considerable time to both configure and get into the game and PrDoom will probably only appeal to die-hard Doom fans.
If you loved Doom when you were younger, then chances are you'll really enjoy the enhancements that PrBoom brings.