I have found that these tools and apps have helped me and my team with collaboration with iPhone development and other software projects.
Having the right tools and knowing how to use those tools effectively can make or break a team, and define whether or not the application is successful or not.
Version Control
First, the utmost tool/application that should be used is Source Control. I can't stress Source Control enough. Use some form of Source Control even if you're a one-man team or the only person working on the project.
I would highly recommend http://subversion.tigris.org/ it is platform independent and is highly customizable and flexible.
Defect/Bug Tracking
Second, your team is going to need some formal way of logging defects and tracking bugs/software issues. Having a bug tracking system in place allows your team members to be on the same page as to what is known as issues within the application. It also allows better management and assignment of defects/bugs which results in a higher level of productivity.
If you're a small team and don't want alot of overhead for a bug tracking system. I would recommend using http://www.mantisbt.org/
Standardize Documentation & Software
Third, having your team come to an agreement of what type of documentation and software creation tools you decide to use is a major issue that should be addressed before any type of work is invested into the project. I would suggest to always go with an open-source, open-format that is available across different platform O/S's.
Hope this list helps... as always you should always use tools and software that best suites your needs. These suggestions don't apply to everyone. But as a startup company keeping costs as low as possible, and using software that results in a more productive environment is always important.
Good luck !!!