Can you afford to lose your memory cards? If they're blank, you can probably find a way make do, although it's probably a hassle. But if they're full of images, it's quite another story. Those little rectangles of plastic, metal, and electronics hold your hard work.
As I've written before, SD cards are pretty resilient little things, but they do break, and they can get lost. CompactFlash cards are also pretty solid, but you can get sand and grit in the sockets. So it's good to look after them rather than just tossing them in the bottom of your camera bag.
I've tried out quite a few memory card cases over the years and have settled on two. Ideally, I would have one case that serves both needs perfectly, but in the meantime, this combination is what works best for me.
Think Tank Pee Wee Pixel Pocket Rocket
One is for when I need to carry a lot of memory cards. When I'm on multi-camera time-lapse shoots, I might easily have 12 or 14 cards on hand. Most of them are SD cards, but I also use CompactFlash cards. So I need something that handles both.
For that, I use the ThinkTank Pee Wee Pixel Pocket Rocket memory card wallet. It's made of a strong, flexible fabric that wraps over itself and closes with a velcro strip. The wallet is not waterproof and is open at the ends.
It's very light and fits comfortably in a pocket. There's also a loop for attaching to a lanyard, keyring, or attachment in your camera bag. And it comes in bright colors, which makes fishing it out of the depth of a camera bag easy.
The cards slot into clear plastic pockets, which makes it simple to separate full cards from empty ones--I turn full cards face-down and can see which is which through the plastic.
There are 4 CompactFlash pockets and 3 SD pockets. But the beauty of the soft pockets is that it's easy to stack 2-3 cards in each pocket. So in practice, I can comfortably fit 12 SD cards and 2-4 CompactFlash cards. And the cards stay securely in place whether I have one card in each pocket or multiple.
Gepe Card Safe Extreme
A downside of the Think Tank wallets is that they don't always provide the level of protection I need. If I'm kayaking in Antarctica or hiking in the thick African dust, I want something that'll keep the elements out. And for that, I use the Gepe Card Safe Extreme.
These things are tough. They're waterproof, crushproof, bulletproof, and bombproof. Okay, so I made up those last two, but it wouldn't surprise me. They're the Pelican cases of memory card holders.
Well, actually, that's not entirely true. Pelican does make their own memory card cases. But you have to choose between the one for Compact Flash or the one for SD cards. Their cases are not designed to carry CF and SD cards in the same case. And I use both types of memory cards.
The Gepe Card Safe Extreme is waterproof to IPX-7 waterproof specification, which means its guaranteed for immersion up to 1 meter. But because they float, the chances of you ever needing a higher waterproof rating than that are slim.
The Gepe Card Safe Extreme holds several different--but not all--shapes of memory cards: MMC / SD, CompactFlash type I & II, Memory Stick, and SmartMedia. For my purposes, the SD and CompactFlash are the ones that matter most to me. There are a total of 4 multicard compartments, with any of the compatible cards fitting in any of the compartments. It holds cards snugly using a stiff rubbery plastic.
It also comes in several colors: Onyx (ie. dark gray), Iceblue, Rosso (ie. red), and Neon. I like the Neon ones because it's both easier to see in the depths of my camera bag and if I do drop it, there's really nowhere it's going to blend in, so I can fish it out of the mud or water quickly.
It would be even better with somewhere to attach a lanyard or keyring, and better still if it held more than 4 cards.