National organization of amateur radio. 
QRZ.com Look up domestic and some foreign call signs. VANITYHQ, a project of Michael Carroll N4MC, is a wonderful site to search for the availability of vanity call signs with specific information on availability by area, by type and by date. I used it to search for my own call sign (K5NOW). There is also a lot of general information available for ham radio too.
This is a wonderful site for reviews of amateur radio equipment. I use it to check out equipment that I'm interested in purchasing for myself. It's interesting how some people love something while others loath it. Use the information to give some insight before making that decision to purchase something. Use it see problems you might run into with what you already have. 
K1IW Amateur Repeater and Broadcast Transmitters Database Websearch. Use this form to generate a list or amateur repeaters and broadcast transmitters to your specifications.
The database covers the fifty U.S. states, the District of Columbia, and the ten Canadian provinces.
Interesting data on amateur radio. Lots of interesting statistics. 
This is an excellent source of radio frequencies used by city, county and state agencies. There are also amateur frequencies used for emergency work. This site is a wealth of information. 
National SKYWARN web site. Maps of watches, warnings, advisories, weather radar sites, surface and wind analyses. 
Look up those 1 X 1 special event call signs (Example: W5T) 
Federal Communications Commission Universal Licensing System home page.