The Digital Library Federation's Aquifer American Social History Online project has released the MODS and Asset Action Explorer.
Here's an excerpt from the announcement:
The service allows anyone to upload MODS XML files, including modsCollection files, and verify that those records comply to the MODS XML Schema and also to check the uploaded records against the Aquifer project's MODS Levels of Adoption Guidelines . . . In addition to MODS records, the service also allows the upload of Asset Action Packages . . . which is another experimental format being developed by the DLF Aquifer project. An Asset Action Package is an XML file containing a defined set of actionable URIs for a digital resource that delivers named, typed actions for that resource.