There's another letter-writing website out there, but it exists specifically to connect other letter writers and provide pen pal connections. I already correspond with a number of members, and although I proceed with caution with anything really new and anything free, if you're looking for more pen pals yourself, you should check it out.
We Love Snail Mail
I just joined up in order to have a look around (heaven knows I don't need more pen pals!!) and it seems pretty cool. I've already complained vociferously to its powers that be about the required entry of your birthdate in order to join - but I've been told it's a required element of the pre-packaged software, so there's no way around it. I just lied about mine.
I haven't explored it much, but again, I'm not really seeking. If you're itching for more letters to write, I can vouch for quite a few of the existing members there that they are already loyal and dedicated pen pals, so go have a look. It's free!