I have this one:
http://www.ilounge.com/index.php/ipod/review/10269/
I really really like it. I'm not one to strap my phone to my belt, I like to keep it in my pocket, so this is perfect. Great protection, without too much added bulk. It is vented at the bottom for the speaker/mic and is open at top for access to the sleep/wake key and headphone jack. I picked it up at my local AT&T store for $19. I'm very happy with it. I will be adding some sort of screen protector as well though, Apple sells a nice set of two for $15.
