Ask HN: free online tool for project coordination?

We started working on a new open-source project with a group of enthusiasts, and we need some common place to discuss the product design and to record the project progress.

What are the best free tools available for this?

I tried trello.com, but there's a problem with browser compatibility, and also it's not so much transparent for this purpose.

The source code will be published on bitbucket or github later. At the moment we're in the conceptual design phase.