Mike,
Are you just focused on collaboration or are you looking to build a sustainable knowledgebase?
We looked at Chatter - its a cool tool - but went the route of a wiki because our goal was not only to collaborate, but to build, as an internal community, a shared knowledgebase of re-usable content. We went with Confluence as a hosted wiki and are happy with the results. Not sexy but works well ... kinda like those of us on this list?! Anyway, a repeat endorsement for Confluence. Highly recommended.
Malcolm