The harp is best appreciated in a chamber-music setting, and the works by Debussy (sonata for flute, viola and harp) and Ravel (Introduction and Allegro for string quartet and harp) are justly famous. I also very much enjoy Arnold Bax's Phantasy Sonata for Viola and Harp, as well as the chamber works by Toru Takemitsu -- which traces its lineage to both Debussy and Japanese traditional music.
There are some great compilations of harp concertos that have your usual suspects (Handel, Mozart, Rodrigo, etc) covered. As for out-of-the-way works, I highly recommend the concerto by Alberto Ginastera: a latin-american infused work, pretty modernist, but is very exciting and enjoyable.
For folk harp, you can hardly get better than Patrick Ball, whose metal-stringed harp has a sparkling, bell-like tone. The late Derek Bell of the Chieftains handles the music with unusual sensitivity and insight that betrays his classical background. From Scotland we have William Jackson and his evocative original composions, while the exponent from Wales would be Robin Hwu Bowen.
Finally, if you like new-age music, the album Moai by Monica Ramos and the eponymous album by Emer Kenny are both beautiful and fun. Hillary Stagg has been mentioned before, and I'd say the best place to start is The Edge of Forever. Get the double live CD of Andreas Vollenweider if you have the chance.
Edited by FalconP - 10/27/11 at 8:55pm