At the moment, I see the following sites, to name a few:
And there is a new site, mycallbot.com, which references those shown above and a few more, callhunter.com, numberinvestigator.com and phoneowner.info.
mycallbot.com includes a database which crossreferences a caller-id to multiple numbers, so that you can find alternative calling numbers before they hit you. Very cool. I had received calls from two numbers, in different area codes, for the same new pest. Using the info at mycallbot, I found eight more of their numbers and added them into my phone system's database. Sure enough, the next day, one of those called me and was blocked by the computer.
You can generally find these sites by simply entering your suspected junk caller's number as a ten-digit number (without spaces or dashes) in Google's search box. If it's junk, other people will probably have reported it already and you can click through from the Google search results to these sites and add your own report.