Three levels, not really complete. (They're not playable, anyway - no weapons/ammo/baddies, really.) They were made to inspire mapmakers, according to the author - and there's some interesting stuff here in level design. Check 'em out, see what you can make of 'em!
Hard to know what to say about this one. It's more of an experiment than anything else. Mapmaking at times is beautiful... but there are a number of problems with the map (some delineated in the readme), and finishing it can be very difficult. (I never did find a save term, so when I got stuck in a poly well into the map, my only choice was to quit and start over... very discouraging.) Still, there is quite a bit to look at here - if you're building maps, this is probably worth a run-through, just for ideas.
This was FUN. 8 or 10 levels, depending on how you count... beautiful mapmaking. Some of the levels had a flow problem (hit this switch, run over here, look for a newly-opened area... run BACK over here, hit this other switch, lather, rinse, repeat), but overall, this was acceptable - the levels were just very nice to look at. Lots to kill, lots of ammo to kill it with. Terms were translated from the original german, and can be a bit difficult to follow (especially since they (for the most part) rely on words, not pictures, to get you from point a to point b)... but again, this isn't critical - when in doubt, kill stuff and throw switches. Give it a look - mapmakers, there are lots of cool ideas here; players, there's a chunk of fun carnage here.