Baresip is a portable and modular SIP User-Agent with audio and video support. Tries to be a Swiss Army knife for SIP and VoIP. Supports encryption. Has an embedded web server with an HTTP (REST?) API for controlling the utility. Modular architecture.
See your server in a web browser and perform system tasks with a mouse. It’s easy to start containers, administer storage, configure networks, and inspect logs. Full dashboard. Lets you configure things as well as look in on them. Extensible with a plugin architecture. Packaged in many distros, including Debian. Does not replace the shell, does not take over the system's configuration so you can work from both ends and they won't conflict. Doesn't step on other management apps, either. Uses your account, relies on sudo to elevate privs. Mobile friendly. If you can reach it using SSH, you can admin it.
Jarvis is an ultra-light and multilingual voice assistant. Designed for home automation, it can run on energy efficient systems (eg Raspberry Pi). Very easy to use, it leaves you the choice of recognition engine and speech synthesis and installs them for you. It is extremely customizable, including via the addition of plugins. French and English.
A modular Python framework for implementing chatops bots. Aims to make it easy to write new plugins that implement various skills and interfaces. Supports XMPP MUCs. Can be configured from inside of chat, so you don't have to edit a config file and restart the bot. Implements command access control.
Hashkill is a FOSS password cracker that uses the OpenSSL libraries as its back end. It uses plugins to implement different password types, hashes, and even file types (like passworded .zip files). It's even CUDA aware and can use Nvidia GPUs to accelerate password cracking.
HybridAuth is a library for web apps which implements social sign-ins (meaning, you authenticate with an existing social network account (like Twitter or Facebook)) rather than applying for an account with an e-mail address. Lets the user import established profiles and contact lists from the socnet account they logged in with. Also has a library of plugins for existing frameworks and applications, from Drupal to Concrete to Wordpress. Written in PHP so that it'll run on just about every webhost out there out of the box.
Err is an extensible XMPP bot framework written in Python. Can connect to XMPP servers just like a client and communicate through them. Implements basic IRC client functionality, can connect to MUCs (Multi-User Chats) on XMPP servers, and supports plugins to extend the functionality of individual bots.
OSclass is an open source clone of Craigslist that aims to be simple and easy to set up and administer. It is claimed that it has a management back-end that's even easier to work with than that of Wordpress. Written in PHP, uses MySQL. Themeable, has a plugin system.
A simple looking kanban written in PHP. Has the usual reporting and analytics features (including charts and graphs). Supports exporting stuff to different formats. lightweight. Supports plugins and integrations. Aims for simplicity. Multiple views, search function. Easy enough to run solo. Uses markdown. Use email to start new tasks in a project. XMPP and Slack aware. MFA enabled! Has its own API (https://kanboard.net/documentation/api-json-rpc).
A web based XMPP client. Designed for multiuser chat. Tries to be be easy to customize and run. plugins and events. mobile aware. github repo. Join multiple MUCs, multiple private conversations. Notifications. Moderation mode. Requires that the server have HTTP bindings enabled.
Software for composing music on a Linux box. Seems to have a tracker like interface. MIDI enabled. Seems to have a synthesis feature in addition to using samples. Can consolidate sequences into single objects. Embedded emulation of some of the greats, from the 303 and 803 to the 6581.
Personal bookmark service. Does not use a database. Can be run entirely privately. Has an API and a full-text search function. Very simple to use. Also lends itself to snippets, note taking, comments, hashtags. Offers RSS and ATOM feeds.