Is there a source port that...

[Skatepunk60]

opens doom in a window so you can do other things WHILE playing doom? Thanks for any info

[Fredrik]

ZDoom is one that can play in a window. PrBoom is another one.

Not sure how you're supposed to do something else while playing the game though. I think all DOOM source ports require focus to accept input, so you can't really write a document with the keyboard while playing with the mouse or something like that :P

[NiGHTMARE]

Eternity is yet another one.

[DooMBoy]

I believe EDGE supports playing in a window, though I could be wrong :)

[Draconio]

AFAIK, Legacy allows windowed mode as well. Heck, I think nearly all Windows ports do.

[Erik]

I often have a 30nm or something else running in a prboom window while doing other stuff.