The Muse's Music Hall Cardshop
A really great place, not even commercially operated: you can send cards with a selection of music, choose from lots of pictures, and write your own message. An advantage: the recipient doesn't need a password to pick up their card.
Commerically operated, but lets you send cards for free. Lots of cards to choose from. You can't write your own message, but just like the real cards for sale, you can choose from lots of poetry for various occasions, and these come with music also. Again, no password is needed to pick up a card.
Need to send a love letter but not quite sure what to say? Let Cyrano help. Letters also available to help you dump someone. You can also write your own card if you'd prefer.
One of my personal favorites: Xena postcards! Choose from a huge variety of pictures of Xena, Gabrielle, Lucy or Renee, artistic cards (ie, not photos like the others), Solstice cards, scenes of Bacchae, and more. Recipients do need to enter a password, but where else could you send someone a Xena card?
Has a small but nice selection of postcards, including holiday cards, and lets you write your own message. In addition to the PrintMaster site also has a free graphics gallery so you can download all sorts of clipart and graphics and stuff.
Haven't really checked this one out yet - if you do, let me know how it is.
The Virtual Florist and Another Flower Place
If your tastes run more toward flowers than cards...