Is there an open source x file loader/saver library around? I haven't had any luck in google.

edit - saving is important BTW.

For 3D models, Assimp will read them in, but it doesn't look like there is a .X exporter yet.

.X is not the most complicated format in the world, at least for the plain-text variant, so you could write one up. http://paulbourke.net/dataformats/directx/

i believe the june 2010 dx9.0c sdk includes some .x format code samples - but don't hold me to that. i just read .x. blender or truespace writes them for me.

i remember reading about this

http://www.toymaker.info/Games/html/xanimator.html

The x file format is seemingly deprecated and I recommend you avoid it and move to something else like FBX for example, unless ditching it is not practical for your particular project. You will generally find more support with another format. This thread may be of interest.