Author Donna Gephart has a helpful guest blog from veteran greeting card writer Sandra Louden on how to write cards, plus card markets beyond Hallmark and American Greetings (they don't accept freelance submissions).
I also blogged last year about Linda Fulkerson's excellent primer on how to break in writing greeting cards. She attended the Erma Bombeck 2006 workshop and picked up detailed insider tips, including the best type of cards to pitch and how to present your card ideas.
Some Greeting Card Markets Open to Freelancers:
Blue Mountain Arts wants submissions about "friendship, family, special occasions, positive living" and other topics you might find in a greeting card. They pay $300 for poems used in greeting cards or $50 if it's used in an anthology. They are hosting a poetry contest with a June 30, 2008 deadline. No entry fee. First prize is $300, second is $150, and third is $50.
Melting Pot Gifts publishes cards with interracial themes.
Papyrus Greeting Cards accepts both artwork and writing (scroll down page for writers' guidelines). No info on pay rate. From their website: "Our text complements the variety of looks we offer: Fine art, whimsy, traditional, contemporary, photography and humor. Traditional or sentimental text is most often used in seasonal captions (Christmas, Easter, etc.). Papyrus also needs contemporary text that expresses common sentiments in fresh, conversational ways. Keep these texts short, simple and meaningful. The best rule for contemporary greetings is: Avoid cliches."
Warner Press publishes christian themed greeting cards (scroll down their page for greeting card submission guidelines). The pay $35 on acceptance.
Freedom Greeting Card Co. creates cards for the african-american market. They currently want "contemporary humor for the Cheers line and contemporary, sophisticated messages for Cappuccino and DeCaf lines."