I currently use the Dopp Front Getaway Pocket previously mentioned, and similar to other front pocket wallets from Fossil, etc. It's maybe a layer or two thicker than I'd ideally like, but has served me well for years and I'm in no hurry to replace it just yet.

Part of the trick to a slim wallet is how it's loaded. Under normal circumstances I'll have four credit/debit cards in there, but I keep them stacked face-to-face in pairs in such a way that the raised letters do not overlap (i.e. one card upside-down with respect to the other - and remember, face-to-face). This cumulatively saves the equivalent thickness of about one extra card.

Then I took the four store "loyalty cards" that I regularly carried and slapped them down on the flatbed scanner, getting a clean scan of the bar codes. I trimmed and rearranged these, printing two bar codes on either side of a business card-sized piece of paper. When I get to the checkout stand, I simply hand them this "multi-pass" while pointing to the specific bar code, and the cashier usually instinctively knows to block one bar code with their fingers while scanning the other on that side. No problems, usually they think it's pretty neat.