OpenTAL is a Python-based templating system for manipulation of XMLish data.
The most obvious use is dynamic generation of webpages (similar in spirit to php, jsp and others).
It implements the same language (TAL) used in Zope Page Templates; in fact one of the packages included is a Zope interface to the engine.
See the Zope Book, at http://www.zope.org/Documentation/Books/ZopeBook/current/ZPT.stx, for more information about TAL.
Most of the packages are distributed under the GNU GPL, version 2 or later, with one single exception - the "pax" package is public domain.
Disclaimer: the "Open" in the name refers to its open architecture, meaning it is designed to be easily extended. It does not refer to "open source"; the developers of OpenTAL do not endorse "open source". See http://www.gnu.org/philosophy/free-software-for-freedom.html for why.