M.L.
The two suggestions you've been given are great ones! I also love the Jacquie Lawson cards (www.jacquielawson.com). They are NOT free - you have to pay for a membership (I think it's something like $18 for two years). But I find that a wonderful bargain, because I can send an unlimited number of cards.
I don't rule out snail mail entirely. For some people, and in some circumstances, it's better to send a real (not virtual) card that the person can keep and look at again and again. Many people tell me that receiving something other than junk mail and bills in their mailboxes these days is a rare and glorious treat! But, alas, the cost of buying cards does add up. The postage is a better bargain, economy-wise, than the card itself.