Highly recommended! Rated 5 out of 5 stars
If I had to pick just one extension, I would pick Adblock Plus. Ads are far too often obnoxious and annoying. AdBlock Plus removes that garbage and makes the web much nicer.
Many filter subscriptions are available, so it's all basically automatic. (There is no need to manually select and block ads.) You just pick the subscriptions you want, and the list of ad filters updates automatically.
AdBlock plus can also be used to find URLs of images, video and other files which are loaded by Flash or other plugins.