These are the tools that have been coming in handy during this time:
The upfront costs for these were essentially zero since I already had a hosted domain, my Apple TV has been in my lounge for well over a year and Google Docs are free with a Google account.
There are a lot of wikis out there, but Doku Wiki does not require a database, so there are no additional costs to hosting it. Since my website host charges extra for databases (as do all others I am aware of), this was a very attractive option for us. Setting up Doku Wiki was just a matter of unzipping the archive to my server, setting some passwords and making the data directory writable by the server, very simple. This gives us a common place to share the ideas and documentation on the game design. The wiki page is pretty much the 'source of truth' now for all information.
Screen sharing! This is probably one of the best inventions out there, every time we are having a design session, I have my laptop's screen shared to the Apple TV so whatever notes are taken or whatever wiki page we are looking at is instantly available on the big screen. We don't have to be cramped behind a desk trying to look at a computer monitor!
Now that you can create drawings in Google Docs, creating mock ups of the game screens is so easy! I actually got the idea for this after reading Sarah's blog entry and I don't think I will be going back. It's free, it's shareable and it's very easy to use. Going from a pencil and paper mockup to something we can host on our wiki is a matter of minutes.
I've used tools like Trac previously for issue tracking and integration with version control, but Git Lab takes it to a whole new level. It's just so easy to define milestones, create issues (bugs/tasks) and see what everyone has been up to. Setting this up was not a problem too, my web hosting provider already had Git Lab running, so getting started was simple. It is a bit of a shift in how you work compared to CVS and SVN, but it's definitely worth it.
With a bit of careful planning and use of free tools out there, it's quite easy to create an environment that makes it simple to share ideas, especially in a way that everyone can see and contribute at the same time.