Android Apps to Extensible

TagSpaces is a versatile, open-source application for organizing digital files across multiple platforms using a flexible tagging system. It allows users to manage documents, notes, photos, and more directly on their local storage or synced cloud drives, enhancing discoverability without relying on central databases or proprietary formats. Ideal for individuals seeking robust offline file management and knowledge organization.

K-9 Mail is a free and open-source email client for Android, renowned for its focus on privacy, security, and extensive customization options. It supports multiple account types including IMAP, POP3, and Exchange.