XnaContent is a wrapper that allows a developer to build XNA Content Pipeline files (with an .xnb extension) from outside of Visual Studio. It supports any non-parameterized importer/processor as you'd expect, and the wireup is fairly simple and, perhaps more importantly, is typesafe; if Visual Studio can't find your importer or processor, you aren't going to build your application.
While XnaContent works (I use it myself, this is an extraction from a personal project), it does have some limitations. For example, if you need parameterized content building, like for DXT textures, you'll need to manually specify the importer. The good news is that the TypeMapping system (which uses regexes to match file patterns to filenames) allows you to provide pretty expressive rules and yours are checked before the built-ins so it's easy to override the default "this is a PNG, build it as a Color texture" behavior.
It's important to note that the Build() method is embarrassingly synchronous. This is intentional. I use it in an asynchronous manner, wrapped inside of a Task, but I didn't want to force consumers to necessarily do the same (though maybe I should have!).
Questions, comments, suggestions - feel free to email me or to file an issue on Github.
-Ed Ropple