In a sentance, it's Quake combined with Serious Sam, and put on methamphetamine. This WOULD make it utterly unremarkable, except that you can edit levels on the fly. Just press E, and you can edit the level, save it, and go on your merry way. It also has a surprisingly large number of included maps and multiplayer modes including a co-op edit mode, where EVERYONE can edit the map.

Website: http://www.sauerbraten.org/
Everything you need to start modding maps: http://www.sauerbraten.org/docs/editing.html

Enjoy.

aka minecraft mk. 7

Note:

http://www.moddb.com/games/platinum-arts-sandbox

Looking at the screenshots, it looks like one is a modified version of the other. Either Cube is a modded PAS or the other way around. Even the font and reticle are the same.

Sandbox is based off the Cube 2 engine. :B

Thaaat would do it.

I've been playing this with my friends for a year or two. We used to have this huge collaborative map, made out of our individual houses, ammo stashes, deadly traps in said piles, towers built to snipe into said houses, and lava pits in said sniping towers.
Good times, good times.

This looks like fun, I might make some stuff for it.

I like the original Cube better because it doesn't crash every 5 minutes for absolutely no reason.

But no uh Cube is fun. I wish they would make a multiplayer "SPDM" mode where you have to survive an onslaught of monsters. Co-op mode on some of the SP maps would be fun.