Harp is described as 'The static web server with built-in preprocessing' and is an app in the Development category. There are more than 50 alternatives to Harp for a variety of platforms, including Mac, Windows, Linux, Online / Web-based and Self-Hosted solutions. The best alternative is jQuery, which is both free and Open Source. Other great apps like Harp are AngularJS (Free, Open Source), GitHub Pages (Freemium), Neocities (Freemium, Open Source) and ExpressJS (Free, Open Source).
PowerShell Pipeworks is a Framework for making Sites and Services with Windows PowerShell. It is designed to do for web development what PowerShell has done for IT development -- make it easy to build complex solutions out of simple parts.
AmplifyJS is a set of components designed to solve common web application problems with a simplistic API. Amplify's goal is to simplify all forms of data handling by providing a unified API for various data sources.