Dropbox. Think of it as your virtual hard drive. Dropbox gives you 2GB of storage space on its servers, which you can access from any computer. The service also makes it easy to share presentations and spreadsheets.
Basecamp. Beloved by techies and virtual companies, Basecamp is a project management tool that helps users keep track of shared tasks, due dates, and time spent working.
Yammer. This is a dead-simple collaboration tool that many companies use as a virtual water cooler. Yammer resembles Twitter, but it is totally private, so that only your employees can see what coffee shop you are working from or what you thought of the latest episode of Lost.
Gmail and Google Docs. Google's suite of office software is easy to use, can be accessed from any computer, and -- well, you know where this is going -- is totally free.