I've made a C++ library that imports and exports REXPaint files.
I'm personally using it to load REXPaint art files to display in a game I'm working on, so I figured I should share it.
There's an existing C++ library that does this by GamePopper (and I am certainly indebted to him), but I wanted something that was simpler, faster, and more focused. REXSpeeder's now also one of the few existing libraries that support saving .xp files.
At the moment the library only supports Windows, as REXPaint is Windows-only.
Edit: Now supports Linux and Mac OS X thanks to TheBracket