Osiris is a set of XNA 4.0 Content Pipeline extensions. At the time of writing, Osiris contains:

• A GeoMipMapping Terrain Processor. It is applied to a bitmap containing height data, and will generate all the data required for geomipmapping. At runtime, use the TerrainComponent to manage level of detail.
• A TIFF texture importer.
• A RAW texture importer.

Until there is an official release, grab the source - either using the Downloads link or through git.