Boy, Google's adding alot of cool stuff...just noticed this one:
Google Video (http://video.google.com)

Here's a whole bunch of extremely useful things you can get Google to do for you:
Advanced operators cheat sheet (http://www.googleguide.com/advanced_operators_reference.html).

Well worth bookmarking and refering to.

The whole Google Guide (http://www.googleguide.com/) site is worth a trawl through.

There is tons of stuff/services from Google to explore:
More Google Products (http://www.google.com/intl/en/options/)

Edit : Don't forget to check out the Labs link.

I've been doing this "Da Vinci Code Quest on Google" competition. That introduced me to the Google Book search. That is one fancy tool. It had me searching/reading through some museum catalogue from 1890!

Here's one I forgot about, it's been around for ages, but it's not on the list of stuff at More Google Products:

Google Zeitgeist: Search patterns, trends, and surprises (http://www.google.com/press/zeitgeist.html) (see the 2005 Year-End Zeitgeist and Zeitgeist Archive links in the upper left for some interesting info.)

Their latest homepage addition is Google Trends (http://www.google.com/url?sa=t&oi=promos&cad=hppweb0%3Aen_extra&ct=pro&cd=1&url=http%3A%2F%2Fwww.google.com%2Ftrends&ei=uVpjRLOYHruwiwG6uumLCg), which seems to be similar, but just presenting the info a bit different.

I see GoogleMoon (http://moon.google.com/) is also not on the list.

Anyone know of any other cool, neat or even trivial stuff Google's got going on unadvertised?