The product is available from code.mdsn.microsoft.com, and is available via the Microsoft Public License (Ms-PL)
My favorite command is Collapse Projects
The description from the PowerCommands home page describes it as: "This command collapses a project or projects in the Solution Explorer starting from the root selected node. Collapsing a project can increase the readability of the solution. This command can be executed from three different places: solution, solution folders and project nodes respectively."
However the commands "Close All" and the "Remove and Sort Usings" features are a close second and third.
At the time of this post, the current version was 1.1. You can get a complete list of functionality in the product from the ReadMe file here.
Here is the link to PowerCommands on http://code.msdn.microsoft.com/:Power Commands on MSDN