We pretty much have this site to thank for getting us re-obsessed with the Baby-Sitters Club.
If you're trying to complete your collection, this is a great site to look for people to trade with.
This site has tons of BSC scans, including book covers and pictures from the movie and TV show. chickenqueen must have worked forever on it, and it looks great.
This site by miss_myu has some interesting comparisons between the American and UK books, but the best part is definitely the Unsightly Cover Extravaganza.
A BSC-themed blog by cassandraclue
Raina Telgemeier wrote graphic novel versions of Kristy's Great Idea, The Truth About Stacey, and Mary Anne Saves the Day and this is her website. She also has a Livejournal account about the graphic novels, bsc_graphix.
This site has some basic information about Ann M. Martin and the Baby-Sitters Club.