uBlock: a fast, lightweight, and lean blocker for Chrome, Firefox, and Safari.
|uBlock: an efficient blocker extension for your browser. Fast, potent, and lean.|
Safari: available to install from the homepage.
Opera: Opera shares Chrome's underlying engine, so you can install uBlock simply by grabbing the latest release for Chrome.
uBlock has tooltips throughout its UI to help you along. But just in case you need it, here's a quick guide for basic usage.
On average, uBlock really does make your browser run leaner. 
 An overview of the benchmark is available at this wiki page.
uBlock is also CPU-efficient
Details of the benchmark available in this LibreOffice spreadsheet.
Being lean and efficient doesn't mean blocking less
For details of benchmark, see uBlock and others: Blocking ads, trackers, malwares.
Some quick tests:
These tests are by no means complete or comprehensive, but do remain helpful.
To benefit from uBlock's higher efficiency, it's advised that you don't use other blockers at the same time (such as AdBlock or Adblock Plus). uBlock will do as well or better than most popular ad blockers.
It's important to note that blocking ads is not theft. Don't fall for this creepy idea. The ultimate logical consequence of
blocking = theftis the criminalisation of the inalienable right to privacy.
EasyList, Acceptable Ads, ABP Anti-Circumvention Filter List and Cryptocurrency (Bitcoin) Mining Protection List are enabled by default when you install uBlock. Many more lists are readily available to block trackers, analytics, and more. Hosts files are also supported.
Once you install uBlock, you can easily un-select any of the pre-selected filter lists if you think uBlock blocks too much. For reference, Adblock Plus installs with only EasyList enabled by default.
Feel free to read about the extension's required permissions.
Some users might want to check out uBlock Origin: a noteworthy personal fork of uBlock from @gorhill with a slightly different featureset.
uBlock is a general-purpose content blocker, which means it can be used to block ads as well as other forms of content on webpages. uBlock can also be used to help users neutralize privacy-invading trackers. uBlock blocks ads through its support of the Adblock Plus filter syntax. uBlock extends the syntax and is designed to work with custom rules and filters.
** Please note recent news about the status of the uBlock project as of June 22, 2018.**
Acknowledgment: uBlock comes with several filter lists ready to use out-of-the-box (including but not limited to: EasyList, Peter Lowe's, several malware filter lists). We deeply appreciate the people working hard to maintain those lists which are available to use by all for free.