I had been using TDO mini forms (free) plugin for quite some time, and it worked reasonably well, for our 12000 members. It has some limitations/bugs, and I think it is no longer really supported. Now I have switched to using Gravity Forms, (not free), it is perfect for subscribers to post from the frontend, and even has payment options, (which we use to charge users/subscribers for posting to certain categories), though it is quite expensive with the Paypal addon.
Joe