With the advent of credit crunch, and the potential sale of the business I'm working for we have started going down the road of open source.
My first project was a chat server (for which I used openfire), this is now up and running and has been approved after proof of concept to be rolled out to selected individuals within the business.
Second project was a micro blogging site, for this I used laconica, although in it's infancy is a very good product.
My only bug bear about open source the is the documentation.
My current project is to find a sharepoint alternative, I'm currently looking to implement alfresco.