I'll start it off with a couple of recent discoveries, which I will put in quotes, as it's about the closest thing we have to a decent visual separation tool. I will continue to list those posted by others, as well. Please note that not all of these are especially useful or necessary, but if posters like 'em for whatever reason, then I'll include them. I do not, however, condone ad blocking plugins, as many sites rely on them to survive. Removing ads is like file leeching or generally any other kind of leeching where you live off the good will of everyone else and can very well be sucking the life-blood out of a site. We already have pop-up blockers. You don't need to strip ads out of sites as well.
Download Manager Tweak
An extension that gives you more options in the download manager, such as launch, clean up, and open containing folder. Basically gives you all the functionality that Firefox offers in an easier to use manner. Highly recommended.
Can be used in conjunction with Download Manager Tweak.
Download Statusbar is a browser extension that allows you to keep track of ongoing and completed downloads in a hide-away statusbar. Allows you to se the progress of downloads in the browser window and not the other window. Also allows you to right-click files to open them etc. Really good extention in grassmunk's opinion.
DictionarySearch provides a very easy way to lookup or translate a word in a webpage in a user defineable online dictionary. Simply select the word for which you want to know the meaning. Then right click and select "Dictionary Search for ...". A new browser tab or window will open with the meaning or translation of the word you selected. This makes it very, very handy when you want to double-check that you spelled or used a word correctly in a post, when you come across a word with which you're not familiar, etc.
The really great kicker about this? It works in text area fields!
(Description from home page) The ieview plugin is a simple Mozilla and Mozilla Firefox extension (for Microsoft Windows systems), which allows the current page or a selected link to be opened in Internet Explorer. I use Mozilla 99.99% of the time, but there are those moments -- particularly when testing new pages, or when viewing that rare IE-only page that's actually interesting -- when I need to see what things look like in IE.
Scratching the itch, this plugin adds menu items to the page context menu, and the link context menu. Right-clicking a link now includes an "Open link target in IE" menu item. Right-clicking elsewhere in the main body of the page (not within an image, text box, etc.) gives "View this page in IE."
In both cases, we try and track down the IE executable on your system, and launch with the appropriate URL. Brain-dead simple, but I'm using it daily.
Amusing little plugin that changes the name of the browser in the title bar and other places. Good for tricking IE users when used in combonation with an IE theme.
Change what your browser reports it and the OS are to the server. Great for spoofing using IE when a site "requires" IE but really doesn't. Remember to set it back to Default when you're done getting around some stupid IE-only restriction.
A Gmail notifier that integrates into the browser's user interface.
Cute menus... adds icons to the right click menu... looks great!
Tab Browser Preferences
An extension that lets you change options that deal with tabbed browsing. You can do things from making all links open in a new tab to making bookmarks overwrite the last tab.
Like the already mentioned IE View, only it covers a hell of a lot more apps.
Show Old Extensions
Really quite useful. Allows you to see pre 0.9 extensions in the extension manager. You can then access their options etc... instead of having to wait while the authors upgrade.
Adds a "Remove this object" entry to the right-click context menu, which will remove an object from a webpage temporarily. The effects can be undone by reloading the page.
The Image Toolbar provides quick access to some simple image functions and is highly customisable. If you are unfamiliar with the concept used with IE's Image Toolbar, I'll explain: You hover your mouse cursor over an image and a toolbar is displayed in the top left hand corner of the image, which allows you to save the image and perform other functions.
Each button's default function can be set for a normal click. Holding CTRL when clicking the button will activate the other function available.
Save - You can set this to either prompt you to save or to automatically save to a previously chosen directory.
Copy - Let's you copy the image location or to copy the image (so that you can paste into an editing program).
Image Folder - This allows you to open your designated image folder quickly or change it to another folder.
Add Bookmark Here
This extension adds an option to the bookmarks menu and/or the context menu for bookmark folders that enables you to add a bookmark to that folder. This feature is present in Opera where it is called "Add page here..." and I think it's a very convenient way to keep the bookmarks sorted.
Advanced Highlighter Button
Same as Google toolbar's highlighting feature. Unlike the emulated Google toolbar, though, this works with the standard search field; therefore, it only adds one button instead of a large toolbar. It also adds some additional features that make it superior to the Google toolbar. It work's on multiple tabs, going back and forward doesn't erase the field or deactivate the highlighting, and an enhanced visibility option will bold and /or enlarge the size of the highlighted text.
WeatherFox uses data from The Weather Channel's international weather service to display your current weather and up to nine days of forecasting. It can be positioned on your Status bar, menu bar, location bar, or personal toolbar. It supports multiple languages, Celsius and Fahrenheit, and can display weather for anywhere in the world.
The follies which a man regrets the most in his life are those which he didn't commit when he had the opportunity. - Helen Rowland, A Guide to Men, 1922