We'd be lost without these at Utility World:
Airliners.net
Put in a registration, get back photos of the airplane. They have just about every aircraft flown by major airlines. Good for finding out type of aircraft heard on the air, or just to see who's talking.
AirNav
Searchable database of navaids and aiports.
Selcalweb
Searchable database allowing lookup of aircraft registration from selcal, or vice versa. Won't let you download the whole database and search it at home, but those who want to manage a file of 19,000+ entries can do that at another UK site.
NOAA Weather Theme Page
Gateway into the vast amount of weather product made daily at NOAA.
FCC AM Query
Useful for IDing AM DX, or finding source of co-channel interference. Searchable on a number of fields, returns either a little or a lot of highly technical information.
ITU Ship Particulars
Not the best database of ships, but the only free one. Search on vessel name, radio callsign, or MMSI. A different database maintained by a private party specializes in Russian merchant ships.
QRZ Amateur Database
The free search on this commercial site simplified ham radio QSLing and general IDing of stations by about 1000%. However it's also good for IDing amateurs only heard on the air. Put in a callsign, get back the FCC record.