For the certain levels there are .map sources available. You can either go to the subpage of the level you’re interested in and get its source separately or you can just grab source packs from this page:
Q1SP levels sources:
Solarfall, The Castle of Koohoo, Nastrond, Adamantine Cruelty – download
Q1DM levels sources:
Uzuldaroum, Aghast, Zed II, Lilith, Zed, Hook In Mouth, Ferrum, Selfburning, Ricochet, 3 or 4 – download
Q3 levels sources:
Khaooohs, Nemesis Online, Uzuldaroum III – download
It turned that all the custom made levels you can see on the right sidebar made me working as a professional level designer at Nival in the past and now in the game dev department of Mail.ru. I work in the game industry for over 8 years already and participated in developing games like: Firestarter (as a freelancer for GSC), Silent Storm Sentinels, Blitzkrieg 2, Night Watch, Heroes of Might and Magic 5, Allods Online and game prototypes for PS2 and for Unreal 3 engine. Now I’m working on a MMO Skyforge.