While searching for new editors to try out, I came across this one new awesome map program. It works like a breeze and I ended up with a full-blown single player episode in no time!
Seven maps of progressive size and difficulty, following a psychedelic texture theme (you might have to play through the first couple of maps until you've picked up on the vibe). Should be cool for speedrunning too.
Download (Run as a mod, then load "e1m1" and have fun)
Thanks to Andrew Apted, leileilol, QMD, MadFox, Brian J. Peppers and all others I forgot.