Steve ran an excellent session last night on Pentatonics. Using this simple five note scale (major scale without the 4th and 7th) in improvisation gives solos a modern and very melodic feel. When played over certain chords it can also give an "out" feel. For a full breakdown check out this link.
http://www.scribd.com/doc/1011599/Pentatonic-Scales-for-Jazz-improvisation