I posted before about the CRM Metadata Browser that can be used to see and document the various CRM entities. Another similar thing available is the ability to get the XSD schema for the CRM entities. As you drill into an entity from the metadata browser, you can simply change the directory in the URL or just type it in as follows:
http://yourcrmserver/sdk/xsd/entity.aspx?entity=account
Of course, just change the entity listed at the end with any valid entity schema name. How's that for cool?