I am currently evaluating wicket for the use of a rich intranet application, and I am very curious if there are big projects/websites which take the use of the apache wicket framework?
There are only a few mentioned on the wicket homepage. (Beside that, I am only aware of a big german publisher for scientific literature which uses wicket for their homepage)
The Wicket wiki has a list of Websites based on Wicket and a (few) User Stories. I spotted some pretty serious websites in this list but I'll let you judge by yourself.
http://mobile.walmart.com is probably the biggest one I've heard of.
I know of a big German online-bank that uses wicket. I can't tell you the name (or I'll have to shoot you).
You can look on job boards who is searching for Wicket-developers and you will perhaps find some big players that use Wicket.