Xmllint

Synopsis
xmllint is a command-line validator for XML.

User commentary
Please sign all comments.

Source code and licensing
open-source

Support for TEI
Sebastian Rahtz reported on TEI-L, "the RELAXNG support in xmllint has historically had issues with TEI schemas. I never got to the bottom of it, but certainly it does not always work. I don't have any workaround to suggest, apart from saying it may work with a simpler schema."