I would like to know if a particular company uses Google Adwords – I am researching their ads. Is there some kind of directory for Google Adwords users?

There is no directory for adwords users. Most sites have to put "ads by Google" by that section of the page (but major players like AOL, CNN, etc. don't). You can also right click on the ad's title and see what URL it goes to under the "properties" tab…this often has the word "ggogle" embedded among the URL parameters.