Writing about web page http://www.citeulike.org/
Looks pretty good to me. It's limited as to which providers it works with, but it covers some of our biggies including JSTOR, Ingenta and ScienceDirect.
It seems like its related to the whole Web 2.0 thing that I posted about earlier today, since the tagging system could be one of those "folksonomies" that I keep reading about.
I think its got potential, but I'd be interested to hear if there are any academics out there using it in anger!