I just got wind of this lovely project from The Letter Writers Alliance.
Caveat: I am posting about this before I try it myself, but it sounds pretty cool. Have a look at Random Notes of Kindness. In a nutshell, you create a kind note, mail it to the PO Box of the project coordinator, and she will mail it ahead to someone else. You include your own name and address on a separate piece of paper, and she will mail something to you from another anonymous sender. She requests that you include a 44-cent stamp, if in the USA.
It seems like a very new project, but a lovely one. There is also a Random Notes of Kindness blog. For anyone who is looking to both send and receive more mail, but doesn't know to whom to send, this might be a great place to start.
An official description, from the Random Notes of Kindness blog:
Random Notes of Kindness is a project to collect and distribute hand generated notes of kindness through the use of social media.
The Notes of Kindness will be generated by hand and sent through the mail to a special post office box where I will collect them. The Notes will then be resent randomly on to those who are in the pool of having originally sent notes. The original sender's identity is kept anonymous throughout the whole process.
The Notes become property of "Random Notes of Kindness" and may be published through a blog, book and other media outlets. All senders' and recipients' names and information will always remain confidential and will not shared with any other person or entity.
If you do participate, or have participated, let me know how it goes!