AutoPatchWork: for paginated web pages, this magically makes the page into an infinite scroll, preloading the next page as you get close to the bottom. Great for reading stupid NyTimes articles that get paginated into 8 pages. Also forum threads, search results, etc. I've never had it do anything to screw up a page, although it has occasionally failed to trigger, but that's no worse than what I was previously doing. This extension has simply saved me a ton of time and helps me use the mouse less.
PDF/Powerpoint viewer: Converts all links to a .pdf or .ppt to link instead to google docs' inline viewer. I hate waiting to load a large application every time I click on these links, this extension makes my experience much better. Doesn't work if the document can't be fetched from the web (password protected, internal network, etc), but in those cases you can right click and just load the large application instead.
Google Translate: Auto-detects if the page you are looking at is in a different language and offers to translate the page to your language of choice (english for me).
Pandora: - Not the official Pandora extension, that one is pretty worthless. This one integrates your pandora controls into your browser. Only useful if you use Pandora of course.
Favorite Doodle: Change google's homepage doodle to your favorite on days when there isn't a special one running. I wrote about this in 2009.
Secbrowsing: Chrome itself is known to be very secure, but many of it's plugins (PDF, Flash, Java, etc) aren't as secure. This extension will monitor to make sure that you've got the most secure versions of all of these plugins and helps you upgrade them if you don't.
Goo.gl Url Shortener: Click button, get short URL. From the fastest/most reliable url shortening service. I'm biased though.
23++: If you use 23andMe, adds some much needed enhancements to the website like telling you automatically which surnames match for relatives.
What are some other good ones I haven't tried? I'm always looking for good extensions, especially those that operate in the background making my life better without me having to be aware of them.