I'm particularly fond of open C tuning: CGCGCE

I usually capo up two frets so I'm really playing in D. It works very well for a variety of D fiddle tunes such as "Arkansas Taveler", "Angeline the Baker", "Soldier's Joy", etc. It can even be capoed up to the seventh fret to play in the key of G, which makes a nice counter to someone else playing in standard tuning or open G.

The same tuning works well on banjo (without the two bass strings, of course).