It's a Firefox extension that allows you to customise individual webpages so they work how you want them to.
To get started, grab the Greasemonkey extension, install it and restart Firefox.
Here's what I've installed so far:
Gmail Preview Bubbles: view email conversations at the right-click of a mouse button.
Gmail Persistent Searches: save searches, like Outlook's Search Folders.
Flickr All Sizes menu: quickly jump to the image size you want in Flickr.
Inline MP3 Player: play MP3s directly from your browser without having to download first.
Any other essential scripts I've missed?