0publish makes creating and modifying Zero Install feed files easier. It can create a new commented template for you or edit an existing one.
Various editing operations are available, the most basic being to let you edit it in your preferred text editor. It automatically removes the signature when loading an interface and creates a new one when writing it back out. It will also validate the new version before saving it.
Documentation and download instructions
See also the .NET version of this program, for use on Windows.