While not strictly a wallet, I've been using this as my wallet for over 10 years. I bought two of them just in case one broke, and I've yet to touch the second one. At $2.00 each (when I purchased it) + $1.61 for shipping, I figured why not just get a backup.
It's slim enough to keep in my front pocket. It is the size of credit cards inside with a small leather divider on the inside. It's pretty water resistant and obviously genuine leather. Plus, the zipper means you can keep coins (if you're in Israel or some other country that uses coins more) in it without them falling out.
More importantly, the guy who runs (ran?, I haven't checked recently) the company is really honest and nice. That's why he only charged me $1.61 for shipping: that's all it costed. He even sent an old fashioned cash register receipt along with the official credit card receipt (like from an in-store terminal).
My wallet holds about 15 cards and 2 bills. I'd guess the max is about 20.
http://www.nstarleather.com/optimizer/product/912.html#.UT60bNGc5NM