I gave a presentation at my local CFUG yesterday called 10 Underrated Functions in ColdFusion, I plan on doing one for tags as well at some point. Here was my list of functions, in no particular order:

ValueList - Returns the contents of an entire query column in a string list.

ColdFusion MX 7 added a new function called IsValid. The IsValid function performs data validation just like the CFPARAM tag, and supports all the new data types in cfparam (see my previous post) as well.