2) A Scott Joplin Rag Rhapsody by Kevin Olson.
Actually, I'm currently playing this piece with three other pianists. We learned it awhile back and are picking it up again for an annual studio recital. It's a very fun piece; I like the arrangement very much. (It includes parts from "The Entertainer," and probably from other Joplin pieces as well.)
Two other pieces we've played, both arrangements:
1) Danse Macabre by Saint-Saens
I really enjoyed playing this piece. The individual parts are very simple (sometimes just repeating the same note or octave or chord), but the piece itself is beautiful. It isn't difficult, but it was enjoyable as an ensemble piece.
2) Polacca Brillante by Carl Maria von Weber
Though I like the other two quartets better, this piece is a little more challenging for technique.