Adblock Plus

This is a given whether using Firefox or Google Chrome. I prefer it to Ghostery as it blocks ads at the domain level rather than as elements in the DOM , Ghostery seems to have a negative effect on performance too.

Once you add Adblock Plus you can select a variety of filter lists that block all or specific objects such as trackers, ads, nuisance scripts etc. The most comprehensive one to use is Fanboy's list which catches most stuff you encounter on popular sites.

Reddit Enhancement Suite (RES)

If you are a redditor like myself, then I recommend you get the Reddit Enhancement Suite. It provides an enhanced UI when browsing Reddit. My favourite features of it are that it embeds images and videos in posts, allows you to customize the favourites bar, tracks new comments made since you last visit a post.

EnhancedSteam

As a PC Gamer, you will have no doubt used Steam. The website is great but with this addon, you get a lot more out of the store page than what Valve show you by default. My favourite aspect is the price tracker for games, which shows you pricing trends and lets you appreciate if you are getting a good deal or not from Gaben.

FoxyScrobbler

FoxyScrobbler is great if you don't use Spotify or Grooveshark for you music listening. FoxyScrobbler scrobbles all your track listens from Youtube, Bandcamp etc to Last.FM adding to your music stats.

Other notable mentions: