User Agent Strings. Tons of 'em. Browser, bots, RSS readers. The list is ever growing and never ending. Did you know that "a new user agent string is born every 65 seconds"? *

Here are two extensive lists of user agent strings:

1. http://www.tnl.net/ua
2. http://www.pgts.com.au/pgtsj/pgtsj0212d.html

Very useful if you are spoofing user agents with a tool like the User Agent Switcher extension.

* 96.7% of Internet statistics are made up.