I'm very pleased to say that HubMed now has links from abstracts (only those with DOIs, like this one) to Scopus, Elsevier's new reference linking service. Scopus is similar to, but a huge improvement on, ISI's Web Of Science. For each article, it shows the list of cited papers as well as all the citing papers, allowing you to follow trails of citations in both directions. It also shows other articles related by shared references, and contains links to the fulltext articles on publisher's sites. Another big feature is the inclusion of OpenURL linking outwards from Scopus: once libraries have registered with the service, their users will be able to see links from articles on Scopus to providers chosen by the user's host library. There is also the possibility for Scopus to automatically search the user's library's holdings catalogue and display an icon indicating whether a particular article or journal is currently available. Scopus also provides email alerts for new articles that cite a particular paper, includes web search results from Scirus, and exports citations in RIS format (though without including the extra citation data, unlike WoS).
Scopus isn't going to be free - I'm currently accessing it through our institutional trial - but could quickly become an essential literature search tool.