AirCasting
platform for gathering, visualization and sharing of environmental data
alonetone
home for musicians, their tracks and albums
Amahi
your personal cloud and media server
Autolab
course management and auto-grading of assignments
Barong
auth server
Brimir
an email helpdesk
Concerto
digital signage system
Diaspora
community-run, distributed social-network
Diveboard
Diveboard.com
Duckrails
tool to mock API endpoints
Email Dashboard
email suite for marketing and CRM
Envizon
network visualization and pentest reporting
exercism.io
code practice and mentorship
Expertiza
peer-reviews app, supported by the National Science Foundation
Fae CMS
another CMS
FederalRegister.gov
daily journal for the United States Government
FoodSoft
manage food coop
fromthepage.com
crowdsource handwritten documents
Fulcrum
Original project planning
GitLab CI
continuous integration server
Hackathon Manager
hackathon registration and logistics
Hitobito
full-scale group hierarchy management
huginn
open-source IFTTT
JobsWorth
Project management, time tracking, billing
Juvia
commenting server of the likes of Discus or IntenseDebate
Lewsnetter
email marketing application
Listed.to
online publications with automatic newsletters
lobsters
link-aggregation site, inspired by Hacker News
Morph.io
web scraping tool
Motor Admin
if ActiveAdmin and Blazer had a baby
Notebook Gallery
enterprise Jupyter notebook sharing and collaboration platform
OneBody
church management software
Open Build Service
build service for Linux packages
Open Source Billing
Beautifully Simple Billing Software
Open-Source Event Manager
event management tool tailored to conferences
Organization for Transformative Works Archive
archives of fanworks, including fanfic, fanart, and fan vids
Pay
payments engine
Peatio
crypto currency exchange software
planningalerts.org.au
planning alerts for Australians
Postal
mail delivery platform for incoming and outgoing e-mail
PublicLab.org
collaborative knowledge-exchange platform
Racing On Rails
bike racing association management
RailsCasts
RailsCasts.com source code (thanks Ryan)
Reservations
equipment loans and reservations software
samson
Web interface for deployments
Sanataro
household accounting software
sharetribe
Marketplace platform
Staytus
status pages
TheyVoteForYou
Holding politicians accountable
tip4commit.com
reward contributors
Tracks
software for implementing David Allen's Getting Things Done™ methodology
Trado
simple e-commerce platform for developers
Verboice
Toolkit for people who literally "need a voice"
whitehall
government content publishing for gov.uk
WikiEduDashboard
course dashboard system
World Cube Association
everything for the World Cube Association
Xrono
asset and time-tracking
zammad
enterprise-ready helpdesk and customer support system