Alternative to Basecamp/Campfire?

Does anyone know of any good group collaboration software? Something online that a group can log into and see task?

I am looking for great software, or an online portal... possibly a wordpress plugin, that will allow my group of writers and editors to collaborate more efficiently online. Currently we use Facebook groups to deal with our site... but as you may guess that only goes so far.

I have tried apps like Wunderlist but they all seam to fall short. Other tools such as Google task dont do what we need.

I need something that can have a top then comments about the topic. Think Google Wave... to bad they killed it.

We use wordpress for our site so a plugin to the backend would be a godsend. Most importantly the software must be free! We are working on a no budget kinda life.

Thanks guys,