Tony Fournier's Group Harmony site.
Doo-Wop Groups - Biography & Discography - This is Jean-Cristophe Piazza's site, containing (as it says) short biographies of groups, with their discographies (and links to some of their recordings). What the name doesn't tell you is that it's a treasure trove of group photographs.
Doowopheaven is Hans-Joachim Krohberger's blog. Each "article" discusses an artist at length, with photos and discographies.
45cat is a site for record label enthusiasts. Here you'll find hundreds of thousands of label scans, with all the label info printed for you in an easy-to-read format. You can search by either artist or label and all genres of music are covered. It's a site I use all the time. And, if you're into 78s, the associated site is called 78 RPM on 45worlds.
Harmony Haven (Jim & Nikki) - Specializing in information & photos on Vintage Rhythm & Blues and Gospel Vocal Groups.
Lou Rallo has hosted The Big Beat Show since 1981. Lou plays R&B from the 30s through the 60s, with an emphasis on rare material (check out his all-78 RPM specials). It's heard over WRSU-FM (88.7), from Rutgers University, Thursdays at 7 PM. Catch the live streaming audio at WRSU. A single show is archived weekly at the Vocal Group Harmony site.
Charlie and Pam Horner have developed a site for Classic Urban Harmony. You can find out about their R&B presentations, as well as share their memorabilia. Here, you'll find profiles of vocal groups, from R&B through Soul (with some gospel mixed in).
The Yesterday's Memories Music Shop - Eugene Tompkins (he's sung with the Limelighters, the Mellows, and the Morrisania Revue) has a music store in Atlanta, Georgia called Yesterday's Memories (currently under construction). Aside from Gene being one of the nicest people on the planet, there's something about the store's name that I like. You can email him at: Eugene's mailbox.
Lou Sylvani has updated his Times Square Records site. This should bring you back to the days of Slim.
Christine Vitale hosts the Group Harmony Alley, at Fairleigh Dickenson. It can be heard over WFDU-FM (89.1), Sundays, from 7 to 10 (EST). It's also simulcast over the WFDU website. Shows are archived for two weeks.
Jerry Lawson's Site - For over 40 years, Jerry has been the lead singer of the Persuasions, one of the finest a cappella vocal groups of all time. He's recently created a site with the group's history and other projects that he's into.
Human Nature - Can't say I've ever been a big fan of a cappella, but these guys from Australia are phenomenal. Take a listen to their rendition of the Miracles' Ooh Baby Baby.
Lunch Break - Here's another a cappella group. These guys are fun. Take a listen to Old McDonald Had A Deformed Farm.
However, that's it. I know that there are hundreds of a cappella groups out there. Please don't deluge me with them.