PubMed Mashup
I'm a huge fan of PubMed. If you've not visited it, it's well worth the effort. It's the nearest thing we have to a complete compendium of medical knowledge. Most of it, alas, is only at the abstract level, but as open access takes off, PubMed will become the natural gateway to the full texts (and there's a UK PubMed under construction, too).
So I was interested to see this mashup involving PubMed: a Greasemonkey script that adds blog post trackbacks to PubMed. Sounds like a brilliant idea.