User-created levels for Abuse. Single levels are usually loaded with "abuse -f [filename-of-level.SPE]". More elaborate mods may require special installation locations. If you want even more alternate levels you might also be interested in trying fRaBs.
Levels by other authors
"Die Hard" by Thomas W. Kincaid
A linear level where you drop onto the roof of a skyscraper and have to descend. The tilework is rather amateur in places and the gameplay's not that interesting, but the ending is kind of cool.
"MadMike" by Michael Moss
A well-made, tough level with a lot of traps and tight ammo supply. Some sections of the level branch so that you can do them in varying order.
"SPLUNK!" by Protobob
A rather nicely made, if somewhat short, level taking place in a dark, spooky cave. Purportedly one of the earliest custom levels for Abuse ever released.