Most Firefox users are familiar with Adblock, Googlebar, and other widely popular extensions, however, I thought it'd be nice for people to share their favorite lesser-known extensions.

I'll start. Awhile back I installed a Firefox extension called Target Alert because I was sick of following links, only to find that they were MS Word documents, PDFs, etc. that I didn't want to deal with. Target Alert provides visual feedback (in the form of icons) for the target of links on a page. So, next to each non-HTML link, you'll see the icon of the target file type (based only on the extension, so caveat emptor.) You can customize which file types you get icons for, and can even have icons for links which open pop-up windows (target=_blank), links with JavaScript (onClick...) etc.

The best part is that the new version has added the ability to, instead of inserting the icon in-line, have the icon pop up when you mouse over the link, which won't mess with the page's look and feel/formatting as much. Now, when I mouse over a link, I get an instant visual cue that I'm going to open Excel, execute some Javascript, or open a new window.

So, any other goodies we should know about?