Never before have I seen such a vast library... and never before have I seen such a well-documented library. I wouldn't have thought it possible, but it has to be the most complete and easiest to use library I've ever seen. Handles everything from GUI design (all aspects) to database connectivity, TCP connections, threading, OpenGL, and much more.
User community support is quite friendly as long as you don't appear *too* hopeless.
Only negative thing I have to say about it is that it forces my projects to be licensed under the GPL license rather than the BSD license... but I'll take the hit for such high quality.
EDIT: I have to retract the above paragraph - see this link:
Thanks to Benoit for the correction. Basically, this means that Qt allows for several open source license models (BSD included), and by no means forces you to be GPL.