PC World had a nice article back in October called 10 Killer iPhone Apps for Business Collaboration. The article discusses file sharing, and mentions one of my favorite apps, Dropbox. The ability to access every single one of my documents from my phone or any other mobile device is huge - and there are collaboration features that you can use to assign files to team members, or see when a file has been updated or viewed.
You'll also find mention of some mind-mapping tools. I'm not quite convinced of doing mind-mapping on a small screen like you have with the iPhone. I currently use the MindMeister iPad app, and it's a much friendlier environment for doing mind-mapping.
There are also tools that allow you to manage your Basecamp projects - I haven't used Basecamp in some time, but it's a great project management tool - and having a mobile interface to manage projects is a terrific extension.
Give some of the apps in the article a look - they have some great collaboration features.