Here is a great site!  The Unofficial Enid Blyton Homepage. It has LOADS of
information about Enid Blyton's books - including a big section on the Famous Five.  I loved it just for the pictures alone!

Also try the Official Enid Blyton Website.  It's very good and covers lots of stuff.

...and, if you liked the television adaptation of the series, try this site The Famous Five Television Series (Zenith) Homepage

Check out Beech Books and Records for first edition Famous Five books and other good
memorabilia.

Finally, I had to get this one in!  This is my older brother's site, so please give it a look @ Mega Movies On Line (It doesn't have anything to do with the Famous Five, but it's pretty good if you like movies, videos and stuff like that!)