“Groovy Motherfucker”

Wicked-nifty Javascript UI library, generously open-sourced by Yahoo. Using this with Django and Lucene, I was able to knock out a pretty decent AJAXy autocompleting search box within 24 hours of opening my big yap in a meeting and suggesting it as a feature. (The “Design Pattern Library” is also worth a read to anyone who juggles HTML)