These are for Chrome but might exist on others.
UsabilityGo Extensions LinkSimple icon added next to your address bar so you don't have to trawl through the settings menu to access your extensions.
OneTab LinkIf you're a heavy tab user this can be useful, hit the button and it'll close and condense all of your tabs into a list.
AlienTube LinkYouTube comments for the most part are complete garbage, this extension replaces comments with Reddit discussions (if the video was posted there). You'll see YouTube comments if there's no Reddit discussion.
Currently broken due to a YouTube upgrade, see here for fix.Imagus LinkHover over a thumbnail and it will display the full sized picture or video,
very handy.
Adblock Plus LinkProbably the best of these.
Reddit Enhancement Suite LinkDoes a bunch of things, I mainly use it for the infinite scroll feature.
Pretty Facebook Chat LinkI don't think most of the 'features' work but it will let you have Facebook chat windows in any size and on any part of the screen.
Fullscreen Anything LinkPress CTRL and Space (followed by F11) and it will fullscreen anything flash related on the page your'e on. Useful for certain flash games etc. that have fullscreen disabled.
Unspoiler LinkEnter key words (Star Wars, Game of Thrones etc.) and if they're found on the page you're on you're given a warning.
Productivity / Dev StuffSession Buddy LinkSave your entire browser session to use later, allows multiple sessions to be saved so good for managing work/play etc.
StayFocusd LinkAllows you to manage the time you spend on certain sites, fair bit of customisation.
ExampleWindow Resizer LinkWill quickly change the page you're on to a specified resolution (720p etc.) so you can easily see how your content looks on other screen sizes.
WhatFont LinkFind out what fonts are on a particular webpage.