StreetSign is a digital signage system, originally written for the TeenStreet 2013 (Germany) youth congress. It works with a single server, and multiple client computers connect over the network. It is lightweight enough that a Raspberry PI can run as the server for smaller installations.
Mathpump is a simplistic remote collaboration tool. It could be useful for those researchers who tend to think by drawing pictures (e.g. theoretical physics). Bob uses Inkscape to draw a picture, which is incrementally transmitted to the Alice's computer so she can look at it. She answers by drawing her own picture, which is transmitted to Bob. Transmission happens every time the SVG file is saved. The transmission is incremental; what is actually transmitted is the difference between the old version of the SVG file and the new one.
Sanewall is a firewall builder for Linux that uses an elegant language abstracted to just the right level. This makes it powerful and easy to use, audit, and understand. It allows you to create very readable configurations even for complex stateful firewalls. Sanewall can be used for almost any purpose, including control of any number of internal/external/virtual interfaces, control of any combination of routed traffic, setting up DMZ routers and servers, all kinds of NAT, providing strong protection (flooding, spoofing, etc.), transparent caches, source MAC verification, blacklists, and whitelists. Newer versions abstract the differences between IPv4 and IPv6, allowing you to define a common set of rules for both, while permitting specific rules for each as you need. Sanewall is a fork of FireHOL and can make use of existing FireHOL configurations.