Note: this list doesn't include any of my own levels.

BloodBunny's Lair by Bloodbunny
Happy Semiconductor CTF by stripe
Darker Science by ShadowGpW
Swingin' Jazz by Bloodbunny
Alien Sanctuary by RagE
Happy Castle CTF aka JAIL by stripe
Shockwave by ShadowGpW
Martian Megatropolis by URJazz
Knightmare Castle by Unhit
Happy Hell by stripe
Alien Technology by EvilMike
Cavern CTF by Evilmike
ETs Planet 1 by ET (example level for ETs planet1 v2)
Smog CTF by EvilMike
Industrial Disaster by BlurredD
GEMneric CTF by stripe
Happy Hell CTF by stripe
Astroni-Ave by ShadowGpW
Crimson Diamond by martin aka Partydude
Celestial Wargrounds by Enigma
.:GLoWiNG PoiSoN:. by EvilMike
The Fortress of Fate by BlurredD
Surfs up CTF by martin aka Partydude
Slab RatZ by ShadowGpW
The Marshland of Evil by EvilMike
SPACE ISLAND LEVEL WITH ISLAND by EvilMike
Inner Evil by iCeD
Dirty Laundry by Bloodbunny
Lack of Water by FireSworD
The Genesis Reactor Core by BlurredD
This Level is Untitled by EvilMike

Update: added 7 levels