wkhtmltopdf is an open source (LGPL) command line tools to render HTML into PDF and various image formats using the QT Webkit rendering engine. These run entirely "headless" and do not require a display or display service.
WeasyPrint is a visual rendering engine for HTML and CSS that can export to PDF. It aims to support web standards for printing.
eDocAPI is a PDF Generator API as saas platform lets you easily generate PDF documents from templates, html code and URLs through a simple REST API and web interface.
A reliable free service that includes an API and online tool to screenshot websites, as well as a Web Scraper to scrape data directly from the web.