I started using Greasemonkey with its first release and have never looked back.
Reimplementation of Eliazar Parra's fantastic script for adding unobtrusive infographics to community link sites. The original has external dependencies (e.g. jQuery) that this one doesn't. Please note that this script uses Mozilla-specific features and is unlikely to work in any other userscript environment.
See blog post.
Uses Tom Moertel's IMDb Decoder Ring data to automagically add genre ranking-percentiles to IMDb movie pages.
See blog post.
Keeps track of read/unread comments in Reddit discussions. This was initially intended as an experiment in using Greasemonkey with Google Gears, but now I find the site frustrating without it. Alas, of course, the Gears database is stored locally, so it's no good if portability is a concern. Please note that this userscript requires Google Gears.
See blog post.