TLA+ is a high-level language for modeling programs and systems--especially concurrent and distributed ones. It's based on the idea that the best way to describe things precisely is with simple mathematics. TLA+ and its tools are useful for eliminating fundamental design errors, which are hard to find and expensive to correct in code.

Related contents:

• TLA+ Helps Programmers Squash Bugs Before Coding @ IEEE Spectrum.
• Episode #552 - Thinking outside the box of code @ Changelog Interviews.

Open Source ArchiMate Modelling.
The Open Source modelling toolkit for creating ArchiMate models and sketches.
Used by Enterprise Architects everywhere.

• Archi @ GitHub.

NetBox is the leading solution for modeling and documenting modern networks. By combining the traditional disciplines of IP address management (IPAM) and datacenter infrastructure management (DCIM) with powerful APIs and extensions, NetBox provides the ideal "source of truth" to power network automation. Available as open source software under the Apache 2.0 license, NetBox serves as the cornerstone for network automation in thousands of organizations.

NetBox is the source of truth for everything on your network, from physical components like power systems and cabling to virtual assets like IP addresses and VLANs. Network automation and monitoring tools draw from this authoritative plan to push out configurations and monitor changes across the enterprise.

NetBox @ GitHub

A standard Business Process Model and Notation (BPMN) will provide businesses with the capability of understanding their internal business procedures in a graphical notation and will give organizations the ability to communicate these procedures in a standard manner. Furthermore, the graphical notation will facilitate the understanding of the performance collaborations and business transactions between the organizations. This will ensure that businesses will understand themselves and participants in their business and will enable organizations to adjust to new internal and B2B business circumstances quickly.

OpenInddor app aims to render OSM data inside a building in a 3D way.

CARS is an open source 3D tool dedicated to produce Digital Surface Models from satellite imaging by photogrammetry. This Multiview Stereo framework is intended for massive DSM production with a robust, performant and modular design.

Gaphor is the simple modeling tool.
Gaphor is a UML and SysML modeling application written in Python. It is designed to be easy to use, while still being powerful. Gaphor implements a fully-compliant UML 2 data model, so it is much more than a picture drawing tool. You can use Gaphor to quickly visualize different aspects of a system as well as create complete, highly complex models.