First of all, if you're not using Firefox, you should be. It's faster, more secure, and just generally better (hard to quantify, but the base features make it wonderful). The addition of a few handy extensions helps it to approach perfection. Here are my Top Ten extensions for general browsing. Needs may vary, especially if you are a developer, but these are great for everyone.

1. Adblock - Does precisely what it says. The perk is that you can customize what ads are blocked. In other words it needs to be done manually. Yet, the wildcard feature makes it easy to block everything coming from a particular location http://website.com/ads /*
   
   
2. BugMeNot - "BugMeNot.com was created as a mechanism to quickly bypass the login of web sites that require compulsory registration and/or the collection of personal/demographic informationa" Essentilly, it helps you log you in to the New York Times, Washington Post, etc, without the hassle of setting up an account.
   
   
3. PDF Download - Lets you choose whether you want to view the PDF, download it, view it as HTML, or do nothing. This is useful for when you unknowingly stumble into a PDF trap.
   
   
4. Tab Mix Plus - A slew of options for your tabs, unless you're one of those weird people that only uses one or two at a time. My favorite functions are: forcing links to open in new tabs rather than new windows, backing up my sessions in case of a crash, and adding a loading bar to each tab. Here is my configuration: Link
   
   
5. IE Tab - This is for those websites that refuse to support Firefox. IE Tab allows you to open that webpage in IE in a Firefox tab.
   
   
6. QuickProxy - This extension drops a button on your status bar making it very easy to turn proxy settings on and off.
   
   
7. Video Downloader - This lets you download almost any video or flash animation. Not quite a simple Save Target, but it is the next best thing.
   
   
8. Greasemonkey - A script manager for Firefox. These scripts perform various functions and can be individually tailored. My personal favorite is the Dugg Mirror script which adds a link to mirror every Digg article. -- UPDATE -- the old DuggMirror script died, so I changed the link to a new one.
   
   
9. Nuke Anything Enhanced - Gives the power to "nuke anything" on a webpage. This is extremely useful for striping the advertisements (even plain text) when preparing a document for printing. Unlike Adblock, this is only temporary.
   
   
10. Linkifier- This extension converts plain text links into fully clickable ones. This is not like other extensions that allow you to open the link when you highlight it, Linkifier actually changes plain-text links into normal HTML style ones.

Give these a try, they're quite useful. What is your favorite extension?

Update: The BugMeNot link actually points to the extension now. My bad.