I work in the broadcast industry by day, and I found this search tool on the FCC website to be most useful, hopefully it hasn't been posted before, but I didnt find it.
http://wireless2.fcc.gov/UlsApp/UlsSearch/searchLicense.jspif you click on "advanced search" you can search by discrete frequency or frequency ranges. This may not be helpful for pirates, but plenty of unusual transmissions are actually legal. Pretty much any united states licensed transmission should show up here.
P.S this search can return huge numbers of results, especially as frequency gets higher, so be as specific as possible