I'm a big fan of minimal wallets that don't add a lot of bulk to the bulk of the cards/bills I want to carry.

My current favorites are:

The Slimfold Wallet is my everyday.  Made of Tyvek so it's super-strong with minimal bulk.  Holds 10 cards and some bills comfortably.

http://slimfoldwallet.com/?page_id=5

For a night out when I only need a couple cards, an ID, and a couple bills, I use the Jimi. Mine holds 4 cards one side and a few bills in the built-in money clip the other side.  If you get the JimiX version, the money clip moves to the outside giving you a little more room for cards inside.

http://www.thejimi.com/

For formal occasions, I have a black version of this Martin Margiela card holder. I've yet to find a non-leather wallet I'd want to whip out when I'm wearing a tux or a nice suit, but this manages to be less bulky than the typical bi-fold/tri-fold leather wallet. I load it with a few cards and ID, and maybe a couple of tickets if I'm off to the opera/ballet/whatever.

http://www.oki-ni.com/uncategorised-sale/maison-martin-margiela-11-mens-card-holder/invt/mm1250hay/&...