Normalising URIs

One of the things I like most about CiteULike is that it normalises URIs that it knows about, which mean that you can post or or info:pmid/16099373 or even 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.