Watch the Alternate Tunings online guitar lesson by Chris Buono from 60 Electric Guitar Techniques You MUST Know
If the guitar is tuned to anything but E-A-D-G-B-E, then it's an alternate tuning. Granted, when considering alternate tunings simple alterations such as dropped D are usually dismissed for more adventurous tunings such as Jimmy Page's masterful use of D-G-C-G-C-D tuning for "The Rain Song". With that in mind, it's not the tuning, but what you do with it that's important.
Looking for inspiration? Start with Joni Mitchell. No guitar player has committed more of their playing and writing to alternate tunings than Joni. Additionally, the list of players and the cool tunes they've written in alternate tunings is vast, but some guys were really ahead of the curve such as Kim Thayil of Soundgarden.