Right now I've been using Firefox, so I'm posting Firefox addons.

Cheevos for Firefox - You earn achievements for doing different things on Firefox. If you look at the achievement list it doesn't tell you how to get them either, so it's all a mystery haha. It's fun though, because some of them are achievements for using rarely-used things and stuff. Overall a really fun and interesting addon. Installed it this morning, already have 60 points haha.

Cookies Manager+ - Allows you to view and edit your cookies. Useful for trying to look at cookie data, make a page to set cookies, test things through cookies, or other things like that.

XSS Me - Open the sidebar on any website, click a button, and it XSS-tests every form on the page. Extremely useful for checking if your own websites or other's websites are vulnerable to cross-site scripting attacks.

SQL Inject Me - Open the sidebar on any website, click a button, and it SQL Injection-tests every form on the page. Extremely useful for checking if your own websites or other's websites are vulnerable to SQL Injections.

Hackbar - A large toolbar that makes forming SQL Injection URLs simple as 1, 2, 3. I normally hide it until I need it as on my computer it takes up like a sixth of the screen (it's huge) but it's still fairly useful.

Adblock Plus - This should come pre-installed on Firefox. It blocks all ads, and you can choose to ublock them on some sites. For example, if you really like a website and want it's developers to make money off of the ads, you can unblock that site's ads so that they still make money off of them and aren't blocked.

Post your own favorite addons and why you love them! Any browser, any addon!