In A Nutshell
For blocking ads in a web browser, I recommend using the uBlock Origin add-on. If you are an advanced user, consider using uMatrix. (You can also use both at once, see this page for more information.)
uBlock Origin is free, open source, and it has great CPU and memory performance (see this extensive report). It does block Google Analytics by default, and this tends to break many websites. However, you can customize the blocking to your preferences.
uBlock Origin doesn’t support Safari, so Safari users should consider the uBlock Safari extension instead. It isn’t updated as frequently as uBlock Origin, and it doesn’t include an “advanced mode” where you can enable dynamic filtering.
If you are a more advanced user, you want to consider uMatrix. It’s a firewall which works in relaxed “block-all/allow-exceptionally” mode out of the box. Note that this causes many websites to break, unless you spend time configuring it correctly.
uMatrix is available for Firefox, Chrome and Opera.
This is another version of Adblock that includes the “Acceptable Ads Whitelist”.
Disconnect is part of the Abine Blur suite of apps. Personally, I use the Blur apps for masking my identity online.
Disconnect add-on doesn’t block quite as many items as uBlock Origin. I use uBlock Origin together with Disconnect.
Privacy Badger (Coming Soon)
Last year, the Electronic Frontiers Foundation (EFF) released their own ad-blocker called Privacy Badger. It doesn’t come with a singular list of sites to block, but instead blocks domains that are seen across many domains. I tested Privacy Badger and it looks promising, but it not robust solution.