I've created a new page, Math contest problem links. It lists a number of contest sites. Some of the pages have links to lots of other sites. I'll list more as I discover them.
The impetus for this page is that I like to sometimes find an interesting problem to chew on without having to spend time Googling for Math contest sites. I tried, whenever possible, to link to the page that has problems on it, not the home page for the site. So, hopefully I'll save everyone some searching and clicking.