Search your stuff on Google Docs from Spotlight. (I don't use Google Docs, but really like this sort of integration between the desktop and web services.)

Crappy-looking, and it's taking an age to load my calendars (five min and counting) but probably worth keeping an eye on.

"The Google Maps Plugin enables you lookup your Apple Address Book addresses using Google Maps as well as get directions between your Address Book addresses."

Would be handy. If they worked.

Google Earth for OS X

"gDisk is a software that turns your GMail account into a portable hard drive so you can always have your important files accessible accross the Internet." Seems to choke on files above a couple of MB. Update: version 0.2 fixes this for me.

Works with iChat, Adium.