Existing (Open Source) tools at first for sure. Capture the audience, then extend and provide customized solutions / tools. I like zbruhnke idea as an initial project / mastermind group any one can join to get their feet wet if they dont already know a project or have one of their own they want to work on. But initially to get things started I think it would be best to use existing (open source) tools.

Once the choice of existing or new is made then the choice of technology / tools needs to be made. There are alot more existing solutions that meet the working spec needs in alot of differing technologies. So once the new/existing choice is made you really need to focus on tech (.Net, Python, Ruby, Java, etc) as this choice will help inform the number of potential contributers.

<ThoughsOnConcept> I see this working best kind of how Google did in the early days and how I hear facebook currently works. Developers post project pitches to try and gather other people (Devs, PMs Marketing, etc) to their idea, then working together to make it happen. All members of SG would be under a blanked NDA for pitch ideas and each project once started could have their own. </ThoughsOnConcept>