There is TemplateFx (https://templatefx.org), which as the title asks for, is a simple Templating tool which we use at work for various tasks. It is mostly used to generate router configs based on our standard site templates. However, as the Op also mentions a library it might not be what they are after.
2021 UPDATE If you want an open source and non proprietary solution which uses Jinja2 templating which is commonly used with Ansible and other tools then I also wrote JinjaFx which is written in Python and has a web frontend at https://jinjafx.io