This is cool:
Basically, it's a installer builder for Visual Studio with which you can set dependencies on other modules, which are packaged for the bootstrapper (eg: versions of the .NET framework). When your installer runs, the bootstrapper will verify the installation of the component (including version) you need, and if it's not present, automatically download and install it, before your installer even runs.
What's cool about it (versus just packaging in the MSI's for the dependent components) is that you don't bloat your installer very much (bootstrapper is about 500k), but you get virtually the same effect, due to auto-downloading. Very cool.