Harmonics are an effective tool for tuning your bass to itself.
Play one on the 7th fret of your A string, and let it ring while you play a 5th fret harmonic on your E string. You basically want to eliminate any 'waves' that you hear.
Play around on individual strings for harmonics as well. The easiest places to get them are at 12, 9, 7, 5 and 4 1/3. After that you can get them at 2 3/4, 2 1/2, 2 1/3 and 2. In order, the intervals that you get from those harmonics(relative to the open string) are: Root, 3, 5, root, 3, 5, b7, root, 9.
Then you can combine harmonics from different strings to get cool chordal things. Play a G note on the low E, and get harmonics from the 5 frets of your G & D strings. Instant G chord. Play those harmonics again at the 4th fret instead, and you've got a major 7 chord.
Check the links these guys gave you and experiment with it. It's not necessarily applicable in every style of music, but every now and again a 'sprinkle of tinkle' adds to the vibe.