Ben Lzicar turned an old beer keg into a PC case. Conveniently, it comes with a bottle opener. Inconveniently, it does not come with beer. You can view many photos at Lzicar's Facebook page, or a select few at Legit Reviews forums.
Facebook Link (login required) and Forum Link via Geekologie
Anheuser-Busch maintains ownership of all the kegs and recycles the damaged ones in-house. They never sell used kegs for projects like this.
People get arrested at the local metal scrapyard on a regular basis trying to recycle beer kegs, but Anheuser-Busch is particularly well known for insisting on prosecution.