I thought we could use a dedicated thread and directory for these - I wrote a couple and know of at least one more, but if anybody has written a script, I'd be happy to add it to the list.
If anybody isn't sure what Greasemonkey scripts are, they are basically code that runs on top of a website, enhancing it in some way.
For Chrome or Opera, you don't have to do anything. Click the link to the script, and it will install.
For Firefox, you need to install the Greasemonkey extension and restart the browser, first - then click on the link to the script, and it will install it for you.
Both Safari and IE can't easily handle Greasemonkey scripts, so I'm not going to offer any advice right now.
Achievement Checker - 6/4 update courtesy ping - Install - Screenshot
When you view the list of achievements, all the ones you've already achieved will dim out, highlighting ones you need to work on.
Location Checker - by Lord Tweedmouth and me - Install - Screenshot
Likewise, when you view the list of locations, the places you've visited will gray out. Unlisted locations are added dynamically, which gives a more accurate idea of your progess.
Nearest Features - by Poh Boy - Install - Discussion Thread
Sometimes you need to find the nearest Mab shrine or nearest grocer - this script allows you to see at a glance (and click to set your destination) the nearest location that has the feature you need. Alternatively, Poh Boy has a version that doesn't require Greasemonkey on his website.
Skill Queuer - by ping - Homepage
Queues skills for automatic learning.
Glitch Favor Checker - by Lord Tweedmouth - Install
Adds info about the favor you've accumulated with each giant to the leaderboards page for quick access.