EmuNations

QEMU is an open source machine emulator and virtualizer.
When used as a machine emulator, QEMU can run OSes and programs made for one machine (e.g. an ARM board) on a different machine (e.g. your own PC). By using dynamic translation, it achieves very good performance. 

Steam Rom Manager is a bulk game importer and artwork manager for Steam. It's basically an app for managing ROMs and benefiting from what Steam has to provide. This includes using big picture mode, its overlay, controller management layout,  streaming options and much more..

Playnite is an open source video game library manager with support for 3rd party libraries like Steam, GOG, Origin, Battle.net and Uplay. While providing one unified interface for your games, it includes game emulation support, fullscreen mode, themes and much more.

Requires Windows 7+ and .NET Framework 4.6 

WineD3D For Windows is a DirectX 1-11 to OpenGL wrapper with almost full implementation of DirectX used in Wine. Even if Windows supports DirectX natively, using WineD3D can enhance backwards compatibility with older games, especially on Windows 8+ that don't support 16-bit screen modes. Others possible values include eliminating unsupported versions of DirectX, or porting DirectX applications to OpenGL without having to rewrite the rendering code. That said, it's far from perfect... 

Wine (Wine Is Not an Emulator) is a compatibility-layer capable of running Windows applications on several POSIX-compliant systems such as Linux, macOS & BSD.

Instead of simulating internal Windows logic like a virtual machine, Wine translates Windows API-calls into POSIX-calls on-the-fly, thus eliminating performance and memory penalties and allowing you to cleanly integrate Windows applications into your desktop.

dgVoodoo is a wrapper for old graphics APIs to run on modern Windows operating systems. Its applied using Direct3D11 with different types of wrapping outputs:

• Hardware rendering at GPU feature level 10.1 (recommended)

• Hardware rendering at GPU feature level 10.0 (there are some restrictions)

• Software rendering through Microsoft WARP renderer

The API's it currently can wrap are:

• Glide 2.11, Glide 2.45, Glide 3.1 and Glide 3.1 Napalm

• DirectX 1-7 (all versions of DirectDraw and Direct3D up to version 7)

• Direct3D 8.1

You also need the 3Dfx splash dlls for Glide if you want 3Dfx splash screen or watermark (shameless plug) through dgVoodoo. 

• 3DfxSplashDlls.zip

You may also need D3DRM.dll (Direct3D Retained mode) for some games and demos. It's not part of the OS since Windows Vista.

• D3DRM.zip

EmulationStation Desktop Edition is an open source frontend application for browsing and launching games from your multi-platform game collection. It comes preconfigured for use with RetroArch and a number of other emulators. It's also fully customizable so you can expand it to launch other emulators or applications. It has several features such as minimal setup, full controller support, very customizable, Built-in scraper for downloading game info and game media, fully themeable and much more...

AppleWin (also known as Apple //e Emulator for Windows) is an open source 

software emulator for running Apple II programs in Microsoft Windows. 

It support most programs that could run either on the Apple II+ or the Apple IIe. 

For its other capabilities, please check this reference.