I use the Powerslider. It gives good protection but it does make the phone larger and heavier.
Power Slider for iPhone 3G : Incase Product
I'm waiting on the reviews for the Mophie Juice pak air. It's a little smaller.
mophie - iPhone Battery, iPod Cases, iPhone accessories and more! - Juice Pack air - iPhone 3G
I like the extra battery life that these cases offer. I can at least use some intensive applications without worrying about having to plug the phone in right away. Plus from what I've heard these cases aren't any bigger than other protective cases that don't offer extra battery life. If any one has the new Mophie please let me know what you think of it.