Handwriting is a shrinking art. Handwriting letters is shrinking even more. However, a few intrepid souls are keeping this tradition alive. Here are some of my favorite sites for finding people to inflict my writing on:
Sendsomething.net - this beta site has alot of addresses of people to send a letter or in fact, anything to. I know that Leaving a Paper Trail has played around a bit on this site. I have found 2 penpals from there.
The Letter Writer's Alliance is another site to find penpals. They will pair you up with 2 or 3 people and you take it from there. So far, I have 2 correspondents for the LWA.
The Fountain Pen Network is where I first started on pen friendships. These people are obsessed with fountain pens, but, I have made some good friends there.