A few posts back I was very excited about Wikialong, a portable wiki that allows students using Firefox to make comments on websites. It's high on my "coolness" rating! I've since discovered Diigo, which is Wikialong on steroids. A great cross-browser tool, Diigo is multi-functional, allowing users to highlight and annotate text on websites, post sticky-notes, bookmark, save video clips, post to blogs or the web and share. It loads into your browser and even offers a "light" version called "Diigolet" that's less feature rich but easier to use.
In the image below, I highlighted key text, then posted my comment in a sticky note for other group members to see.
Like Wikialong, this would be a great tool for students to collect and share resources as they create wiki projects (or any other project!) Moreover, it supports multiple browsers, unlike Wikialong so if you don't use Firefox, you can still use Diigo.
I think I'd use Wikialong for younger students (say, 4-9), then introduce older students to Diigo.