LetterRep.com is an online letter writing service. They pay $10 per submitted letter that is purchased by a client.
I know this doesn't sound very promising but they have a large list of requested letters you can write to increase the chance of a purchase. Anyone can claim and write one of these requests and the client chooses the one they like.
Another bonus is that your letters can be sold multiple times. So, if you write a letter for a client and it's accepted, it will stay in the archive and can be purchased over and over. You get $10 each time, paid monthly via PayPal. They also pay an additional $10 every time a client signs up specifically to purchase one of your letters.
You can view the requested letters before you sign up. This will give you a pretty good idea of what they're looking for. Currently, the list includes a plethora of generic holiday letters (to employees and such), character reference letters (not sure how comfortable some people might be writing those), recommendation letters, cancellation letters, etc. You can also just write a letter about anything and try to sell it.
I don't have any personal experience with this site but they've been around a while. It might be a good opportunity if you're a good letter writer. Check out the Writer FAQ's for more information.