I always keep a stash of thank you cards on hand--you never know when you'll need to write a short note to someone. Yes, letter writing has sadly gone the way of dressing for dinner, but thank you notes are still ever so much better than, say, sending an e-card. In fact, I would say that e-cards are almost never right and send the message that you are too busy to take the time to thank someone properly. Unless you're sending an e-card with an online gift certificate or need to thank a group of co-workers for something small, think twice before sending one. (Update: That is unless you are just having some fun, in which case these e-cards are brilliant.)
I'm very into red right now and, actually, always. I have a formal style and an informal style of card to suit the mood and/or the message. The gorgeous gilded note-card pictured is from William Arthur and available here. The look of this card is inspiring to me in so many ways. The lighthearted cherry design is by hello!lucky and available here.
A pack of varied cards is good in case you're sending more than one card to several people who work together. This happened to me when I needed to send a lot of thank you notes after receiving flowers and gifts after a hospital stay. You don't want everyone getting the same card because then it's not as special. It's always handy to have a few blank note cards available as well in case "thank you" isn't the exact sentiment you want to convey.
One of the problems of course is that in this day and age a person's address isn't always readily available. Old-fashioned address books are smart and chic too, but really not practical because you can't always take them everywhere with you and then people move so much these days, you'd always be scratching out addresses. I suggest sending out a nice mass email to all your friends asking them to help you update your digital address book.