Here are two more Greasemonkey plugins I highly recommend:

Customize Google: This plugin makes Google work better and adds links to other search engines.

Add Netflix links to IMDB: Links IMDB to Netflix

Greasemonkey is a nifty Firefox plugin that can modify the behavior and appearance of web pages via easy to author plugins.

For example, Jon Udell has written LibraryLookup, a plugin that links Amazon to your library. As you browse Amazon, it adds links which tell you whether the book you are currently looking at are available at the library. If you add a book to your Amazon wish list, the plugin will automatically notify you when the book becomes availabel at the library.

As Jon notes:

Greasemonkey, though, does seem to open up uncharted territory. As I’ve mentioned before, a user of Gmail was able to add persistent search folders to the application. When search folders were added to Outlook, in version 11, everyone made a big deal about it, and rightly so. It’s a major feature. Now I use search folders in Gmail every day, the feature is indistinguishable from the core application, and it was created by a user!

This kind of customization makes the already attractive notion of software as a service even more compelling. Imagine that your company subscribes to a hosted web app, and it’s mostly great, but there are a couple of features you wish you could offer your people, and the vendor isn’t coughing them up in a timely way. In an AJAX world you might have a pretty good shot at doing it yourself.

Greasemonkey provides the ability for developers to shape the web to their will on behalf of users. You could write a search plugin that aggregates the results from a number of different search engines into a page of your own design, for example.

Unlike the standard XmlHttpRequest available in web pages, a Greasemonkey can make cross-domain HTTP requests.

Because the aggregation is done client side, and because the requests cannot be distinguished from regular browser requests, you don’t need a business agreement with the data providers to make this happen.

With great power comes great responsibility. It’s easy to create plugins that abuse the user or the site providers or both. There’s really not much of a security model to protect the user from malicious plugins; it’s only a matter time before we see a greasemonkey trojan horse. (This reminds me a bit of the Outlook scripting before Microsoft’s long series of security patches.)

Simon Willison has an interesting piece on Greasemonkey etiquette.

The amount of innovation going on in with Firefox is impressive. I was one of the original developers on Internet Explorer, so I have a regard for it that others might not, but I have to admit that Firefox represents a strong challenger.