Airport/city codes:
http://www.airportcitycodes.com

Airline codes (alphabetically by airline):
http://www.travelsucks.com/tools/airline-codes.html

Airline codes (alphabetically by code):
http://www.wajb.freeserve.co.uk/codes.htm

There are also many other sites with the same information.

For odd abbreviations I don't recognize, I usually do a Google search on them, perhaps with a few words of context like "airport" or "travel" to narrow it down by eliminating the same abbreviation as used in technology, medicine and everywhere else.