A 'free' button in the toolbar that adds a search filter to only match papers for which the full text is frreely available.
Citation details from PubMed Central, which includes all the citation data from BioMed Central as a subset, on the 'Cites' page for each article. The nice thing about this is that all the articles for which citation data is available are open access, so can be marked as such in the search results. I've got my fingers crossed that CrossRef's Web Services will make much more citation data available when they launch next year, but we'll have to wait and see.
Dynamic loading of abstracts inline, from the 'Abstract' link. This makes such an impressive difference to browsing search results, I was glad to see it also included in an experimental search interface some people from the NLM published recently. They also found that users liked the display of matched MeSH terms, which HubMed has had for a while. I have to say though, those sliders seem a bit pointless for most of the fields they're attached to. Flags And Lollipops gave the authors a bit of a slating too.