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. Note that there are two version of PrBoom. The one provided here has software rendering and is called prboom.exe, while the other one has OpenGl rendering and can be downloaded from the project site.
The great thing about PrDoom is that it's harnessed the power of modern processors 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.