There are obviously more Buster Keaton sites around than are listed here. Due to the work involved in keeping links up-to-date, this list will remain fairly short. A handful of related sites are also included.
 

Buster Keaton

The Damfinos
Located at www.busterkeaton.com, this is the site of the official Buster Keaton fanclub.

busterkeatonfans YahooGroups mailing list
Sign up and share your thoughts on Buster with like-minded folk from around the world. At the time of writing the list has just short of 400 members.

Buster @ the Internet Movie Database
It's not complete and it's not 100% accurate but does include information on well over 100 of Buster's film appearances. Also listed are directing, writing and other credits.

Juha's Buster Keaton Page
A comprehensive listing of Buster Keaton links - far more than I'll ever manage to list here! 

Generally Buster
Buster Keaton fansite which among other things includes transcriptions of a number of vintage magazine articles. 

Roscoe Arbuckle

Arbucklemania
A Roscoe Arbuckle site with other silent content, including Buster film clips.

Charlie Chaplin

The Charlie Chaplin DVD Guide
A UK based site dedicated to Charlie Chaplin video & DVD releases.

Miscellaneous

Silent Era : The silent film website
More than just Silent Comedy.....

Silent DVDs
An index of silent movies available on DVD.

Pandai Online Store
Gifts & Gadgets for all occasions.