When you click the button on a Steam games listing, it will make a textbox list of games you can copy and send to friends!
This is pretty simple.. when you're on a Steam games listing, like "http://steamcommunity.com/id/<username>/games?tab=all", and you click the little steam button on your toolbar, it will create a text box with each game on the page on a new line.
This makes it easier for you to copy and paste your games to friends, or into files for other purposes. It will also list the total quantity of games you own at the very top of the textbox.
This isn't particularly polished. No warranty, express or implied :)