Compared to other lump management programs around at the time it was released, SLumpEd had the advantage of having a tabbed interface, allowing multiple wads to be open at once, and lumps copied/pasted between them. It also addressed various grievances I (and many others) had with XWE, the most used lump manager at the time.
While it was written with cross-platform compatibility in mind, I never really got it to compile in anything but Windows, though it probably could have been ported to Linux/OSX with little effort. The SLumpEd website (in all its neon glory... what was I thinking?) can be found here. I have also put the source code up on GitHub for archival purposes.