Yep I was going to say, ECF plus any other collaborative tools (Trac, Jira ... RedMine! as long as they have a SCM integration, ticket tracking and a Wiki ... the rest is a bonus or fluff.)
You also should implement SCM for product versioning control... immediate collaboration is one thing but historic control of the project is also EXTREMELY beneficial. Git or SVN are both nice. (despite SVN bashing by Linus T)
Quick note:
SubEthaEdit (mentioned below) is like ECF but drawbacks are...
1. Propietory/Closed/$$$
2. Lacks integration.