pdflayer - Free, powerful HTML to PDF API supporting both URL and raw HTML conversion. Unlimited document size, lightning-fast and compatible PHP, Python, Ruby, etc.
HTMLEmail.io - Responsive HTML email templates for startups & developers
PDFShift - Convert any HTML documents to high-fidelity PDF using a single POST request
Templates by Email Monster - Hundreds of free, responsive email templates. Pick and edit.
Tcpdf - TCPDF: Open Source PHP Class for generating PDF documents
MJML - The open source framework for responsive emails