My work just switched to a new provider. Usually the blocked sites are based on lists of keywords like "gaming", "WoW", "Megamillions", "Casino" or "porns".
If one of those words show up in the URL or URI (whats after the .com) it will block it. Sometimes they scan for keywords on the site itself.
This is the keywords tag on the page I am looking at.

Code:
<meta name="keywords" content="Multi Boxing, Multi-boxing, Dual-Boxing, Dual Boxing, Quad Boxing" />
There are ways around this. I have access to an external proxy that I can open an ssh tunnel to. I force my browser to use this instead of the corporate proxy and gateway and get to everything I want.

googling "free web proxy service" turns up a number of links. Depending on your company's implementation some will work for you and some will not.

This is pretty much what I use http://www.damtp.cam.ac.uk/user/jp10...te/web-tunnel/ with a private external proxy