[jquinby]

With a handheld radio and a Technician License, you can get on the air via local repeaters, work the VHF/UHF satellites, tinker with APRS (using something like this: http://www.mobilinkd.com/).

$500 will get you a good HF rig, but as another comment mentioned, you'll need to factor in the cost of a DC power supply, some decent coaxial cable and something to use for an antenna. Antennas can be dirt-cheap - just wire cut to length and lofted however you can.

/r/hamradio is a solid online resource. There's a semi-official IRC channel too.