There are a number of Half-Life inspired case mods out there, so I make no claims to originality here. There are better ones, too. This was intended to be a weekend project...start on Friday, done by Sunday.
I picked up a new cheap case and PSU last week. I only needed the PSU to replace one in my wife's PC, but it was only $10 more to get a case along with it.
A long weekend was coming up, and I was looking for a quick project that would give my PC a bit more life.
A trip to Michaels craft store and $20 later, and I had a rust-finish kit and was set to go...
Click thumbnails for larger versions:
The case was a cheap one, but had a nice front, with screened drive convers over the entire front for nice ventilation. A 120mm blue LED fan also came with the case, and a built-in card reader.
I lightly sanded the case, and started applying the first part of the rust finish. It's basically a lot of iron particles suspended in a grey acrylic-like paint.
Once that's dry, a liquid containing copper sulphate is applied which causes the iron to rapidly oxidize and create a wonderful aged rust finish. I applied the first coat of iron paint unevenly, in the hopes of controlling the amount of rust.
The back of the case seemed to be treated with something, and would not rust, even with a good coating. I ended up painting the back of the case with some acid I had left over from circuit board etching. That weathered it nicely 
The side pieces were initially too rusty for my liking, so I left them out in the rain for a couple hours, and once they were blotted and dried, the effect was a bit more subtle.
The game-inspired stencil was created in Illustrator, and printed as a 1 pt. outline, which I cut out with an X-Acto knife. The pattern was hand-held against the case and lightly misted with Tamiya acrylic paints.
I'm pleased with the final product. I had the option of sealing the finish with a clear coat, but found it darkened the rust more than I liked. The finish seems solid enough to leave unprotected.