The Arachnology Home Page
From the welcome page:
The Arachnological Hub of the World Wide Web for the study of arachnids (spiders and their relatives)
Acari - Amblypygi - Araneae - Opiliones - Palpigradi - Pseudoscorpiones - Ricinulei - Schizomida - Scorpiones - Solifugae - Uropygi ( Thelyphonida )
Basically a portal with lots of links to other sites of interest (including this site).