AdBlock: Blocks ads, even in YouTube videos.
AutoHD for YouTube Specify default resolution for YouTube videos.
Craigslist Preview You will never know how you used craigslist without this extensions. SRSLY.
Ghostery Blocks tracking scripts. From the company: "Ghostery sees the "invisible" web, detecting trackers, web bugs, pixels, and beacons placed on web pages by Facebook, Google Analytics, and over 500 other ad networks, behavioral data providers, web publishers - all companies interested in your activity."
LastPass Password sync/save; secure vault for your information. I like it enough I just paid for the premium service (only $12/year).
Page Refresh Auto-reload pages at variable intervals. Save some strain on your F5 key.
PrivacyFix Consolidate all your privacy settings for various social networks.
Readability If you have some kind of e-reader or smartphone Readability will save articles for you to read later at your leisure. It also reformats the articles nicely.
Secure Shell Developer tool If you do any amount of command line jiggery pokery it's quite nice to have a terminal window in Chrome.
The Tracktor - Amazon Price Tracker A service which embeds itself in your Amazon page and allows you to track things you'd like to buy to see if they drop below a certain price. When that price is reached it alerts you.