Google Apps like Mail, Reader, Calendar and Docs make cover a big part of my daily work flow. This is mostly because I switch between various machines and operating systems on a regular basis and not so much because I think they are better or whatever than the usual suspects on the desktops. These are the add-ons I still use after an extensive time of testing.
Better GMail 2 This one compiles several user generated Greasemonkey scripts into a single add-on: it adds little icons to emails with attachements, shows the amount of unread messages in the tab header, hides unnecessary clutter in the sidebar, adds folders and much more. If you use the new GMail you should definetely have a look at this extension.
Google Gears You might have come across this one already: it adds offline functionality to online apps. Which is indeed quite useful if you sit on a train. This extensions synchronises your online content with your current machine and lets you create a shortcut on the desktop to access the site directly if you wish. The latest addition to the family on Google’s side is GMail (huzzah!) and many other independent online apps use Google Gears as well.
Google Preview It’s a nice little extensions that only does one single thing: add thumbnails of the pages to your google search result. Which is especially useful if you google for something like LaTeX…
Greasemonkey scripts I currently only use two scripts related to google searches: one that adds real time search results from Twitter and another one that adds a sidebar with results from Wikipedia, Flickr, Dictionary.com and YouTube. Unfortunately, the scripts are loaded slightly delayed and make the page layout jump. I don’t find it disturbing but you might. Both work on my machine and didn’t break anything but I suggest being careful with installing scripts. What extensions do you use that I haven’t added to my list?