I have an idea for a site that would be based on users writing a "letter" and then automatically posting it to the site. Not a comment in response to a blog or a forum, but the content would be the "letter" that users write. In real life, it would be like a place where visitors could write a note/letter on a large sticky note and place it on a wall for all to see (tagged, so others could filter out topics, geographical location, etc.).

What would be the best platform to use for such an endeavor? Oh yeah, it has to be free. Can a beefed-up Wordpress do something like that? Joomla!? Let me know if you need more details. I'll try without giving my niche away.

I think many platforms could do this. I'm not sure if any do it out of the box, but I'm sure many can be tweaked so they could.

For example in WordPress you could have users register and allow registered users to publish posts. There are even ways to set it up so the user doesn't see the backend of WordPress. If you didn't want other people to be able to comment on the "letters" then you could turn comments off.

There might be something out there that does most of what you want out of the box, but that will depend on the details. You can tells us what you need to happen without sharing your niche if you want. Just describe the technical details of who posts letters and what happens to those letters, etc.