The truth of the matter is, we use Web 2.0 tools everyday and may not even know it.
In our class discussion we pointed out our one favorite tool, but it is so hard to narrow it down.  Here is a more complete list of my favourites:

Google
YouTube
GoogleMaps
Facebook
Flickr
Allrecipes
Jing
Captivate
Dropbox
Skype

Note:  I keep coming back and adding to this list...I'm going to stop now.