Blakeney Manor - the first TSP fansite I stumbled across, has lots of information on the books and film versions! Also has all of the TSP books to read for free online, which is lovely, especially the ones that are no longer available in print and/or hard to find!  I do not believe they are an active site any longer, but there is still enough to keep a TSP fan busy for a long while!  

The Scarlet Pimpernel from Wikipedia - It's amazing what you can find on wikipedia. This one article has tidbits about the book, historical accuracy, adaptations, real-life tie-ins and more.

Gutenberg Australia - if you don't want to read the books online you can download them all for free as zip files.

Project Gutenberg - this site has only a few of the sequels, but they can be downloaded in better formats and are better proofread than the zip files from Gutenberg Australia.

Librivox - free audiobooks of a few of the novels.  The books read by Karen Savage are especially a joy to listen to.

Zounds, Fern! a lovely new Scarlet Pimpernel fansite.