I'm a grade 10 level adult pianist and play for pleasure. I just found out one of my friends plays the French Horn and went looking for duets for us (not sure what level he is - we haven't gotten together yet).
The Mozart Horn Concerto #3 that I found (which I've always loved) was arranged quite well - lots can be added in after re-listening to an orchestra recording while practicing.
The searching process was a pleasurable surprise. It was easy to find the type of music that I wanted (hopefully the catalogue will grow). Not only could I view the score, but listen to the arrangement as well. The mp3 files were clearly just converted over from the midi, so dynamics were brutal, but at least you could hear what the parts would sound like togther. A really good service to find music that you don't have to practice too much to play.