This post is a simple overview of namespaces you will run into when working with OData in the ATOM format. The list is not comprehensive, but you can get pretty far with it.

http://www.w3.org/2005/Atom
This is the Atom namespace, defined in RFC 4287. It's used for all "regular" ATOM things like entry, feed, link, content, etc.

http://www.w3.org/2007/app
This is the Atom Publishing Protocol namespace, defined in RFC 5023. It's used for the service document elements: workspace, collection, etc.

http://schemas.microsoft.com/ado/2007/08/dataservices
This is the 'data' namespace for OData, defined in MC-APDSU. It's used for property elements. Typically associated with a 'd' prefix, sometimes it'll be the default namespace.

http://schemas.microsoft.com/ado/2007/08/dataservices/metadata
This is the 'metadatadata' namespace for OData, defined in MC-APDSU. It's used for etag, type and null attributes and the count, properties and inline elements. It's also used for all metadata extensions in CSDL, such as MimeType or the FC_* attributes. Typically associated with an 'm' prefix.

http://schemas.microsoft.com/ado/2007/08/dataservices/related/SOMETHING
Not actually an XML namespace, this is used as a prefix in 'rel' attributes in links to indicate that SOMETHING is a linked entity.

http://schemas.microsoft.com/ado/2007/08/dataservices/scheme
Not actually an XML namespace, this is used as the category scheme that identifies that a category term should be interpreted as the entity type name for an entry.

Enjoy!