SmartVideo for YouTube. Definitely improves the experience, and I can finally watch 720p because of it. Videos don't stop buffering when you pause. Lots of options. Firefox version available as well. HTTPS everywhere. A must if you go on public networks sometimes, makes sure sites that offer HTTPS have them enabled, so people can't sniff your data. Hover Free. Hover your mouse over a picture or direct link to see it in fullsize. Super useful for Facebook and websites like reddit. HerpDerp for YouTube. While the name is silly, it has made YouTube a lot more tolerable for me. Turns all comments into herps and derps until you click them. AdBlock, obviously. Just remember to disable it for websites you actually wish to support, and that has non-intrusive ads! Their websites can't run for free! Firefox version available as well. YTshowrating. Shows the number of thumbs up/down in a nice way on YouTube. Facebook full-size profile pictures Vew full-size profile pictures of people you aren't even friends with. StayFocusd Useful if you're like me, easily distracted by the internet. Block pages (except some, like wikipedia) when you really need to study or concentrate. Gmail checker Small icon in your browser that shows how many unread emails you have. Also check out other add-ons by Google! Reddit plugins: Reddit Enhancement Suite, a must! Reddit Companion, also a must! Reddit NSFW links in incognito, in case other people might see your log.
Regarding SmartVideo, Youtube forced me over to HTML5 with no option to go back to flash and I could not get SV to work even though it claims to support HTML5. I'm on chrome on windows 7 FWIW. If anyone else has this problem, check out YoutubeCenter. You'll need TamperMonkey on chrome to install it. It does basically the same things SV does and a bit more. There's a full-fledged extension also, but I've read it doesn't work.