many more that will improve your experience and efficiency; but, start with these.
Embedly - If you live on Twitter.com you can not live without this. Even though Twitter has been integrating more services that allow you to preview content this will take you to the next level. Videos, audio, photos, posts, etc.; you will see it on without leaving the site.
WOT or Web of Trust - It is a simple extension that assists in keeping you away from harmful websites. Not 100% accurate, but it has saved me a couple times. I recommend to change the searching options to not display on search engines and other sites. It reduces its foot print and lets it do one simple task of warning and stopping you if you are on your way to a bad site.
Cloud Save - An excellent extension that allows you to right click on almost anything and save it to one of many cloud based services (full review). If you are on Chrome OS this is a must have.
Google Sidewiki - This was intended to be a social tool to let you leave comments on webpages for others to see. In my opinion it has failed in that regard, although it does have some use. What I use it for is my browsing history. If you install this and activate it all your browsing history will be sent and saved to your Google account. Some may be scared of that, but I love it and have referred to it many times.
Secbrowsing - Ensures your browser plugins are up-to-date. Easy to use and out of the way when everything is good on your system.
Scratchpad - Write notes and reminders. The best part is that it will synchronize and save your notes on Google's servers so that you can access them from all your Chrome browsers.
TweetDeck - A social platform that brings in Twitter, Facebook, Buzz, and Foursquare. The interface and functionality are top notch.