Adblock Plus is one of the most popular and well-made ad-blocking extensions.
The extension removes annoying things like banner ads and pop ups from your web experience. That may seem like a slight difference, but visiting some sites that tend to be bogged down, especially with pop ups, can be liberating. But if you do choose to use it, make sure to "white list" sites you feel don't have an intrusive ad experience — especially those who make their money on advertising revenue.
AdBlock also works with Safari, Opera, and Firefox if you don't have Chrome.
Download Adblock Plus here.