For network engineers or administrators, the tracert command is a common network test and diagnostic command, which is used to determine the path (all nodes) that an IP packet takes to reach its destination. But the problem is that the result has always been returned only in text form, which is not easy to check. If you want to view tracert results clearly on a map and display more information with graphs, then Open Visual Trace Route is what you need.

Mnemosyne is a free, open-source and spaced-repetition (an evidence-based learning technique) flashcard program for Windows, Mac, Linux and mobile platforms, developed by Peter Bienstman from Belgium in 2003, written in Python. It can help you learn as efficiently as possible, i.e.: optimizes your learning process. Besides, it's also a research project into the nature of long-term memory.

Splunk is a top-level distributed data computing platform for Windows, Mac and Linux platforms. It can process various logs or machine data generated by any system, software or device, and provides operations such as real-time data collection, storage, indexing, searching, correlation, visualization, analysis, and reporting in a wide range of scenarios, so as to transform big data into actions and values of the enterprise.

UMLet is a free and open-source lightweight UML drawing tool (rather than a modelling tool with underlying dictionary or directory of reusable design objects) for Windows, Mac and Linux platforms, written in Java. It is powerful and easy to use, can be used as a standalone app, can also be used as a plug-in for Eclipse, and even provides a browser-based online version.

There are already countless chatting software (such as WhatsApp, Skype, Discord, Facebook Messenger), but almost all of them are non-free software based on central servers, and their security and privacy are not guaranteed (any chat history of you will be sold to various companies). Many of them claim to encrypt communication content, even with end-to-end encryption, but when a platform is proprietary, there is simply no way for others to verify that it is as secure and private as it promises to be. Of course, there are some really safe and reliable ones, but they are frequently blocked by governments of various countries and cannot continue (such as Signal, Telegram).