It lets you maintain a set of DHTML scripts which are applied to some or all webpages you view to change its behavior and content. The scripts are written in Javascript. This may not give you a clear idea of what it does; suffice to say it lets you fix things about webpages that you think are broken. For example, I've got user scripts installed to do all of the following:
- ununderline: Convert all non-link underlined text to italics, so only links are underlined
- Linkify: Convert all text URLs to clickable links
- Livejournal userpic adder: shows LJ userpics when you mouse over a
username
- del.icio.us skin 2: different look for del.icio.us
- ALT tooltips: makes Firefox show an image's alt attribute on mouseover when there is no title attribute
- Newsmasher: adds a floating link at the top of every page which brings up a floating iframe containing the del.icio.us posts for a URL, so you can read other people's comments about it
- Friend of Print-Friendly: takes you directly to printer-friendly pages on a whole bunch of websites
- Expand Area: lets you drag the corner of a textarea to make it bigger (this one is a great example of how flexible and useful greasemonkey is)
- Check Range: lets you check a range of checkboxes by clicking one at the beginning and then shift-clicking one at the end of the range
- Salon Auto-Pass: bypasses the Salon daypass requirement
- Metafilter deleted posts: shows links to deleted posts on Metafilter
- SourcePlease: rewrite sourceforge download URLs to point directly to a file instead of their "Choose a mirror" page
- HTTP-Be-Gone: remove the example "http://" from form fields that expect a URL, so you don't have to see "http://http://" after you paste
