Putting a discriminator tap into a scanner can net you a lot of neat stuff.
You can decode trunk data channels with a program like Unitrunker, for starters..
You can decode POCSAG or FLEX pager data (the alphanumeric pagers people carry around), though it's of questionable legality to do so..
You can pick up data signals like "weather fax" images, etc., if I recall correctly. My main use of a tapped scanner is for playing around with trunk systems.
You can also do some neat data stuff without a tap.. a few of us here in Calgary are now monitoring ACARS (kind of a messaging format between aircrafts) with only a cable from the earphone output into your computer's sound card.