CFDump is one of ColdFusion's best tags. Simply give it a variable and it will output the value no matter its type (array, structure, query, cfc, java, etc it can handle just about everything).
Often I will use it to output a scope like form or url or variables (be careful with this one though as all queries are in this scope by default). The one scope I have not been able to dump is var until yesterday when Jared Rypka-Hauer passed along a tip. To dump the var scope do this:
You may have to adjust the output attribute of your functions.
[UPDATE: 9/5/07: This only works on CF 7, click here for how to do this with ColdFusion 8]