One of the things I like most about CiteULike is that it normalises URIs that it knows about, which mean that you can post
info:pmid/16099373 or even
http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6SYV-4FX23FY-3&_coverDate=08%2F31%2F2005&_alid=356223062&_rdoc=1&_fmt=&_orig=search&_qd=1&_cdi=4844&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=0af86d8bdaa4e097bc8ac51186461586 and they'll all be identified as the same object (the first three will be linked by PMID and all four will be linked by DOI). The same applies to books, which are linked by their ISBN.
As far as I know, it's the only bookmarking site to do this.