Doug has given us a great list!
To it I would add Google and Google Images. The latter
provides lots of photos of that suspect, hard-to-identify bird.
Caveat: Not all Google pix are identified
There are also websites dedicated to the identification of
certain groupings of species. E.g. the GULL IDENTIFICATION WEBSITE at http://www.geocities.com/rainforest/canopy/6181/gulls.htm.
(See also links to other websites with large collections of gull photos at the
bottom of the GULL IDENTIFICATION website).
Whenever someone posts a request for help with some ID
conundrum, I get on the internet and look for answers.