Steve Hargadon created this Ning - it's a space where teachers get together and share wisdom, resources and conversation. It currently has over 40,000 educators subscribed. A terrific resource for extending your learning!
Diigo is the best of the lot in my opinion! Not only are you bookmarking, you're creating a community of people interested in the same subject. Text on a site can be highlighted, annotated and discussed using Diigo. It's a perfect way for students to discuss a site's content and for them to share their web with you. Here is the link for creating a free Diigo user account: https://secure.diigo.com/sign-up