I'm using:
uBlock - adblock alternative, more lightweight on the PC and the browser itself, been using it for a while now, have no complaints.
WebOfTrust - I find the reviews and security ratings useful, especially if you accidentally run into a phishing website
Evernote Clipper - If you use Evernote, I'd say this is a must, easy and fast way to keep articles you want to read later on your phone, or to make some eBay bookmarks for auctions with notifications when they're soon to be over
Streamus - Great way to create easy playlists and listen to music with no need for an open Youtube tab. Extension is fast, you can import playlists easy, has a search feature and you can create your own playlist in a second, with no need for signing in with a Youtube account, definitely a recommendation
Imagus - Hover zoom alternative I've been using ever since Opera 12 was really popular, lightweight and reads images well, as well as imgur albums and makes it easy to scroll through all of the images in the album. I use it for reddit mostly, but it's useful on any other site as well