Zest is described as 'Offline search tool for developers. Find what you need without the need for Internet access'. There are more than 10 alternatives to Zest for a variety of platforms, including Mac, Windows, Linux, Online / Web-based and Self-Hosted solutions. The best alternative is DevDocs, which is both free and Open Source. Other great apps like Zest are Zeal (Free, Open Source), Dash (Paid), Velocity (Freemium) and Codespace (Paid).
Zeal is a simple offline API documentation browser inspired by Dash (OS X app), available for Linux and Windows. - Quickly search documentation using Alt+Space (or customised) hotkey to display Zeal from any place in your workspace.
Dash is an API Documentation Browser and Code Snippet Manager. Dash stores snippets of code and instantly searches offline documentation sets for almost any API you might use. You can even generate your own docsets or request docsets to be included.
Snipt is a code snippet service for teams or individuals. Team accounts allow your team members to create and edit private and public code snippets. Search through all your team's snippets, and view detailed diffs of changes to each snippet.