This is a great question, and I think some kind of response should be immediately made and stickied. Perhaps a Map List, similar to the Mod List. It's especially important because, well...
...to my knowledge there isn't a collective site with screenshots easily available. I'm a bit out of touch with things, though, so it might've changed. Try clicking "search" on top-right side of the page (Main Index - Who's Online - SEARCH - Calendar - User List), and looking for maps.
You can also use maps made for Dominions II. Most of them will work straigth away, but due to changes in how transparency is handled some maps are invisible (=unplayable) in Dom3. You have to save the image file (tga) again without alpha channel, or use
this converter program by paradoxharbringer, or download a fixed version (IIRC at least Karan and Clash of the Titans were fixed and posted to this forum a few months ago).
Illwinter's own site has a rather good listing of DomII maps.
http://www.illwinter.com/dom2/maps.html