Delicious (http://delicious.com/) is one site that I will certainly use. It used Captcha, which took two tries, but otherwise, it was easy to sign up. I often want to access my websites from other computers and this could be very helpful, since I have several groups of websites, associated with several e-mail addresses. My delicious page is: http://delicious.com/MaryDen
I can understand its utility in library settings, as well as for other social groups and activities. It seems like a very helpful site with many applications.
Web2Access is very positive in its evaluation of delicious (http://www.web2access.org.uk/product/97); however, since it uses Captcha for log-in, it did not a 100% evaluation for those with blindness/severe visual impairment. The rating for blindness/severe visual impairment was 94% and for partially sighted/visual acuity and visual stress it was 67%. delicious was rated at 100% for those with hearing impairments, however.