Everyone's starting to write about COinS, which is good. COinS stands for 'ContextObjects in Spans' and is a way of putting links in (X)HTML that point to different places for different people.

A ContextObject, part of the OpenURL Framework, contains all the metadata about the thing being linked to (and sometimes the referer as well). When used in COinS, the ContextObject goes in the title attribute of a span element with a class of 'Z3988'. For example (linking to a journal):

<span class="Z3988" title="ctx_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:journal&rft.issn=1045-4438"><!-- COinS --></span>
or (linking to an article)
<span class="Z3988" title="ctx_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:journal&rft_id=info:pmid/15889096"><!-- COinS --></span>
Anyone with an appropriately configured Greasemonkey script, Firefox extension or bookmarklet will then see a link to their local OpenURL resolver (which may be their university or local library, for example), dynamically created within that span.

All of HubMed's abstract pages now include COinS which, when processed, will replace the default SFX link with a link to the user's preferred OpenURL resolver.