The Letter Writers Alliance is really neat. Their blog is way interesting too. You do have to pay a membership fee, but its totally worth it if you want a good pen pal. Plus they always have cool downloads!
Postcrossing is another awesome site. It is only postcards and not letters. Its way neat to receive postcards from all over the world. The way they have it set up is if you send a postcard you get a postcard. They also have a page with all your statistics, so you can see who you have sent to and what countries. I have gotten some way awesome postcards! Its really neato!
The last site that I have had great success with is sendsomething.net. Here you can find people who want pen pals, postcards and other fun mail items! It does take a little bit of searching to find a good one! (but its totally worth it!)