When you say "open source" do you just mean free software, or do you mean "I need/prefer to be able to see the source"?
Note that your decision will be influenced by the nature of your project. There are many free development/project hosting sites that require that your project must be an open source project and free/open to the public.
You may also choose to go with a particular hosting platform based on the language you are using to develop the project. For example, CodePlex (http://www.codeplex.com/) is a site that hosts open source .Net based projects, and Java.Net (http://community.java.net/projects/) hosts Java projects.
The other answers given to your question are solid, here is what I currently use or have used in the past:
A great continuous build tool JetBrains TeamCity. (http://www.jetbrains.com/teamcity/) The tool has out of the box support for many build tools as well as for building Visual Studio solutions out of the box. It is free for teams of 20 or less developers. It also has loads of functionality out of the box, and can be up and running for you in minutes - a remarkably low learning curve without cutting back on features.
A useful SVN repository which is free for two developers, and will save you the time of setting up and administering your own SVN repository is Unfuddle. (http://www.unfuddle.com) Unfuddle also has extra paid-for features and basic task tracking.
Another paid source repository is ProjectLocker (http://www.projectlocker.com) which has low priced SVN repositories and Trac integration for task management.
A useful task tracking tool is Remember The Milk (http://www.rememberthemilk.com) - it does not work on "tickets" like Trac, it is not only for tracking projects, but it does allow you to email each other tasks, and to have shared task lists. I also point them out because the product itself is developed by a distributed development team and you might want to try mail them for advice. :-)
All the best to your team!