Two important aspects about managing multiple projects with multiple teams are scheduling of the projects & milestones and allocation of resources to these projects. Over the past few years we have been using a simple Google Spreadsheet based utility which we had developed in-house to schedule projects and to allocate resources for projects. We had earlier released this as Free Software under a GPL license and made this available as free template and over 1000 users are currently using this tool.
The utility is available as a free Google Spreadsheet Template at http://bit.ly/1b4V7cM. Alternatively you can see a read only copy at http://bit.ly/1coiWyM. The source code for the Google Apps Script part of the application is available at the Zyxware Github account.
It is usually possible to do project scheduling and resource allocation manually without any tool when there are only a handful of projects and a handful of team members. But once the number of projects and team members increases it would become very difficult to do this optimally without some kind of a tool.
There are lot of commercial tools out there that can do this for you but we felt that it just had to be something, that was very easy to use, that provided a lot of visual feedback to make decision making easier, and that allowed real time collaboration. We went ahead and created a Google Spreadsheet with a little bit of automation using Google Apps Script.
We still use this tool internally but we are currently in the process of evaluating other options given that our team size and the number of simultaneous projects we handle are well beyond the numbers which we had thought this tool could handle. We are exploring options not because the tool actually reached its limit but because we are now increasingly becoming aware of a requirement to have more compartmentalized control over teams and units of teams and a requirement to have stronger control over who can do what.
We hope that this utility will help small companies who are trying to find a good, easy to use and free project scheduling and resource allocation utility.