You can get the Case-Mate barely there. It doesn't add much bulk, but honestly it's kinda hard to take off the phone. It's also glossy white, which isn't for everyone.
I personally use the HTC-made case and the Seidio Active with kickstand when I need more protection. The HTC case adds no bulk, and really probably doesn't offer a ton of protection, either.
The Seidio case is pretty awesome. It doesn't add as much bulk as you would think, it has a kickstand (which does protrude a bit from the rest of the case), and it protects the phone from drops.