There are a lot of card drives in the world, but I'd like to highlight this one: Send Kids the World. They focus on getting postcards sent to kids who have life-threatening illness. It's incredibly easy to participate and is completely free. The families don't pay any hosting fees and you only provide the postcards and stamps.
From their website:
Choose a child from the Current Kids list.
Find the address or P.O. Box listed for the child so you know where to send the card.
Write a cheerful short note to the child. For example, tell about the picture on the front of the postcard and the place where you’re sending it from (i.e. Hailey, We live here just below Mt. Timpanogos. We hope Florida is beautiful this time of year! Love, The Smith Family). Or just a simple ‘Greetings from…’. Simple is best. Remember, you’re not trying to cure them or be a miracle worker; you’re just making them happy for a little while!
Put appropriate postage on the postcard and mail it to the address listed for the child.
PLEASE BE CAREFUL WHAT YOU SAY. Know the status of a child before writing your message and mailing the postcard. Don’t write get well soon because some of the kids will not get well. You want to cheer up these children and their families, not add more hurt to their lives.
Have fun sending the postcards!
What a simple way to make a difference. They have lesson plans for teachers too! Please feel free to add other such services in the comments. If you're participating in Month of Letters, here's a good project to help fulfill your requirements.