Pleth has 3 locations (Batesville, Conway, and Jonesboro) being able to manage multiple projects at the same time has grown way past the enormous whiteboard at our Batesville office, and being somewhat OCD, I have always looked at a whiteboard in my office as more of a hindrance than a help anyway. In the past our internal project management has always been up to whoever was heading up a project. Each one of us have our own unique way of management. I am a strong Outlook user and have used Tasks to manage projects that I had going on in the past but my partners have always relied on their whiteboard pretty heavily. We have also used a pretty robust intranet solution from Vialect (which I also highly recommend) to store our client proposals, track hourly time, etc.
A few times in the past I have collaborated or freelanced with other companies on various projects and on a few occasions I have had the opportunity to use email is still the killer application but sometimes shooting interoffice emails back and forth is not the most effective way to communicate. With the message boards inside of Basecamp all messages are displayed in reverse chronological order and the best part, files can be attached to messages in the message board, so this actually makes this part of the application even more collaborative. You can even categorize the messages inside the message board area.