An SSD would be fairly good. You might be able to get your hands on a Samsung 830 128GB (if you can find one) for ~$100, I know the Crucial M4's are going at $110 right now.
Crucial M4 128GB. $110http://www.newegg.com/Product/Product.aspx?Item=N82E16820148442
Other than that, don't really see anything outright beneficial you could do with it.
- Maybe some aftermarket fans, a controller and damping foam, quieten the rig a bit.
- Some LED lighting to make it glow, though from your pic it looks like you already have.
- Squirrel the money away toward a custom water loop.
- Another 8GB of RAM and a $30 sound card. Kinda pointless though.
Or not on the rig at all. Drop the $100 on some new speakers/headphones, mouse or keyboard. Even a new chair or desk.