Well, it all begins with play, of course. Google Docs has been critical to 23 Things. It's the mechanism for producing and keeping track of the survey and the tool we use for tracking everyone's progress with 23 Things.
Google Books has also become an important part of my life, but more on the personal side. I have been using it to get free books to read to my children.
I love Google Scholar. If I may stroke my own ego for a second, what I really love is seeing an citation for an article I wrote back in 2003 that has been cited by 10 other resources that you can find in Google Scholar. Love that.
Google Maps is pretty much the only navigation tool I use these days, especially since I've moved to an Android phone. Essential.
Google Patents is fascinating and can become a huge timesink for curious folks like myself. Just refreshing the front page and seeing what comes up is entertainment enough.
Google Uncle Sam is a huge resource as well. Not something I'd be likely to use often in my position, but everyone should know its there.
Google Sites...I've never known anyone who created a site with it. Not much to say there.
Great stuff overall. In retrospect, I wish we had directed people to Google News. The way it aggregates news from around the globe is just astonishing.