Quite a few, but the one that may be unique is a tool that scrapes the location data from an eBird list of rare birds sightings in my area and pops up a map for me, so I can see if any birds have been sighted near me that I don't already have on my personal birding list.

It only took about 15 minutes to throw together into a bookmarklet, so it was a really low effort, but I use it daily.