OpenMSX

openMSX is a free software emulator for the MSX architecture. It is available for multiple platforms, including Microsoft Windows and POSIX systems such as Linux.
For copyright reasons, the emulator cannot be distributed with original MSX-BIOS ROM images. Instead, openMSX includes C-BIOS, a minimal implementation of the MSX BIOS, allowing some games to be played without the original ROM image. It is possible for the user to replace C-BIOS by native BIOS if they prefer.
openMSX emulates a large amount of MSX systems and MSX-related hardware, including:

MSX TurboR
Moonsound
IDE Controller by Sunrise
GFX9000
Pioneer Palcom LaserDisc

openMSX also emulates the SV-318/SV-328 computers, the ColecoVision and the Sega SG-1000.
Notable features include:

Scalers
Debugging including visual debugging of graphics
Tcl Script Support
Cheat Finder
Game Trainers
Audio/Video recording
Reverse support

openMSX has an open communication protocol to communicate with the openMSX emulator. Utilizing this communication protocol enables to write versatile add-ons for openMSX. Projects making use of this protocol include the following applications:

openMSX Catapult
openMSX Debugger )
openMSXControl plugin
NekoLauncher openMSX
openMSX Peashooter
openMSX Control Plugin for Gedit

There is a dead subproject to redevelop Catapult, a GUI developed for the emulator that is part of the project, utilizing Python and the Qt toolkit.
The separate openMSX Debugger, written in C++ and utilizing the Qt Toolkit is not in development anymore, as efforts are now focused on the built-in debugger.
As of release 20.0, a GUI was integrated that replaces almost all functionality of the Catapult and separate Debugger programs. Both these sub-projects will not be developed further, in favour of the built-in functionality. The new GUI uses the Dear ImGUI toolkit.