I've been juggling for a few years now and find it both useful for zoning out and thinking about tough problems, and intrinsically satisfying - it's a great feeling to nail a new trick for the first time.

For anyone interested in learning I found this video site extremely helpful:

http://homepage.mac.com/abramr/juggling/tutorial/category/in...

Takes you from complete beginner to covering loads of 3 ball tricks, and onto 4 and 5 ball if you are so inclined. Has side by side full speed and slow motion videos.

The mathematical structure behind juggling patterns is quite interesting - various notations have been developed including siteswap:

http://en.wikipedia.org/wiki/Siteswap