Sometimes I send e-cards for birthdays, but for my parents or really close friends who live far away, I usually send a real card in the mail. I know how it feels to get a card you can hold in your hands.
I also think that Thank You cards are appropriate to send through the mail. I have friends who have mailed me thank you cards after a baby shower, for instance. I think that sending a paper card takes an extra step and really shows that you took the effort to remember this special person in your life.