I picked Diigo to use for my Web 2.0 application presentation. The website seems really useful for highlighting the web and sharing web resources with my friends, colleagues, and students.
I signed up for Diigo almost a year ago, but never saw real applications for my students. Now they have Diigo educator accounts so you can use it safely in your classroom. I saw it more for social networking, but after playing with it for a few hours, I like the collaborative sharing piece. I often come across articles/websites that are really cool, but don't want to add anymore clutter to my "Bookmarks" on my desktop at school.
I am still trying to figure out if this will be a website I frequent after my Web 2.0 class is over.
