Collections
AIDS-Selos - The largest collection of AIDS stamps on the web apart from my own. This site is in Portuguese so I have linked to it via an online translator.
Blair Henshaw Collection - The earliest online collection of AIDS stamps. Unfortunately, this collection contains several errors in it, although this may be due to the librarians that maintain the collection rather than the late owner and original collector.
AIDS Posters - A very cool collection of posters about AIDS. Some of these designs are so cool they really should be on stamps.
Hennefeld Collection - Gay and lesbian history on stamps
Ailments of Venus Revisted - An very large award-winning exhibit of syphillus, gonorrhea, and AIDS on stamps. (The link leads directly to the start of the section on AIDS).
AIDS Postcards in India - An article about the official pre-printed postcards about AIDS in India as of 2005 or so.
Good places to buy stamps
Ebay - The world's largest auction site has dozens of auctions of stamps about AIDS at any given time. Some great deals are to be had on stamps if you look carefully. One must use some caution though, as there are counterfeit stamps on there as well.
Delcampe - Outside Ebay, arguably the single-largest auction site of stamps about AIDS. Great place to find related materials such as postcards, telephone cards, and First Day Covers about AIDS.
StampCenter.com - An excellent source of stamps about AIDS and HIV. The site also contains several AIDS-related stamps that are not correctly identified as being part of the AIDS topical list.
Herrick Stamps - Another source of stamps about AIDS (look under "medicine"). This site, unfortunately, has few pictures of the stamps in question.
Philatelic Supplies UK - Another stamp site with many stamps about AIDS. This site has an excellent list of stamps, but only provides a listing of AIDS stamps as part of a greater listing of medical stamps. It has no pictures of the stamps themselves, so know what you want before you buy from here. (This site makes an excellent reference guide to look for pictures of his listings.)
Neofila - A very good site for former USSR and Yugoslovia countries.
Organizations
GLHSC - Gay and Lesbian History on Stamps Club. They maintain an excellent listing of stamps about AIDS. (Their list includes many celebrities with AIDS whom I do not list on this website).
Shout Outs
I want to thank the staff of these websites for their assistance in identifying AIDS stamps.
Sinisa Pavleski's Stamp Site - Website of a friendly fellow in Macedonia that lists regular Macedonian stamps (not the tax stamps, unfortunately). Sinisa helps me translate the writing on Macedonian tax stamps. The pictures on his site tend not to load, so you may need to hit reload several times to get them all downloaded. They're all there, they just don't transmit well for some reason.
Topicals Listings from United Nations Philatelists, Inc. - Thanks to Bill for helping me track down the elusive Lesotho 2001 stamp. This site, while hard to use, is an excellent source of information about UN-related AIDS stamps (World AIDS Day, World Health Day, UN releases, etc.)
Serbian Stamps - Thanks to Konstantin for helping me sort out the mess behind the Serbia and Bosnia-Herzegovina postal tax stamps. This is a very good site for Serbian stamps, although they do not list AIDS-related postal tax stamps. You may be able to specially request them though.
rec.collecting.stamps.discuss - An internet newsgroup where the folks can be an absolute fountain of information when it comes to researching stamps.
Space Covers - Thanks to Joe for his help with the local posts page. Although not his specialty, he was kind enough to do some research for me and the page would not exist without his efforts. If you're in the market for stamp covers with an astronomy or space exploration theme, this is definitely the place to check out.