- AdBlock -- probably the most important one here. I can't stand browsing without it.
- Disconnect -- blocks tracking web page elements from major providers (Facebook, Google, Twitter, etc.)
- Explode -- expands most URLs created by URL rewriters (e.g. TinyURL)
- KB SSL Enforcer -- This one actually causes a bit of trouble, so you may not actually want to use it. It tries to ensure that you're talking to websites over encrypted channels by detecting whether there's an SSL version of each site. Even though it has systems to override its decisions, it still ends up screwing up a fair amount of the time (especially with online banking) since many sites don't actually mirror their SSL sites correctly.
- Keep My Opt-Outs -- opts out of personalized Google ads
Web site integration
- BugMeNot Lite -- makes a little widget pop up in the Omnibox when BugMeNot is available
- Delicious Bookmarks -- bookmarks pages quickly in Delicious. There are many Delicious extension, but I like this one since it shows you tag suggestions.
- Google Docs Viewer (by Google) -- fast access to all your documents, easy to open them in new windows
- Google Voice (by Google) -- shows number of items in your Google Voice inbox, makes all phone numbers click-to-call
- Minimalist for Google Calendar and Minimalist for Gmail -- these two let you disable many individual elements on Google Calendar and Gmail. On my laptop where there's limited screen real estate, this helps quite a bit.
- Google Quick Scroll -- when you search for something and follow a link on the search results, this presents the option of jumping to the location of the snippet on the page
- History of these2 -- lets you search history more easily by time and site
- RSS Subscription Extension (by Google) -- adds RSS widgets to the Omnibox