EmuNations

Ruffle is a Flash Player emulator written in Rust. Ruffle runs natively on all modern operating systems as a standalone application, and on all modern browsers through the use of WebAssembly and has extensive content compatibility in ActionScript; the language that powers interactivity in games and applications made with Flash.

Users or website owners may install the web version of Ruffle and existing flash content will "just work", with no extra configuration required. It will also detect all existing Flash content and this is an entirely open source project maintained by volunteers.

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. 

VirtualBox is a powerful x86 and AMD/Intel64 virtualization solution for home and enterprise usage. It's quite feature rich, high performance yet is freely available as an Open Source Software. While it's mainly a community effort, Oracle is backing it to ensure it maintains high standards of quality.

Presently, it runs on Windows, Linux, Macintosh, and Solaris hosts and supports a large number of guest operating systems including but not limited to Windows (NT 4.0+, DOS/Windows 3.x, Linux (2.4, 2.6, 3.x and 4.x), Solaris and OpenSolaris, OS/2, and OpenBSD.

Launchbox is a front-end wrapper for emulators. Not only does it automatically download box art for games, it also grabs fanart, screenshots, logos and more to create an immersive experience via BigBox.

It also supports arranging and filtering by genre, platform, ESRB rating, developer, publisher, genre, etc. It also includes automated import processes for your MS-DOS games, ROM files, and Steam in order to help get you up and running as quickly as possible. Game box-art and metadata can also be automatically downloaded from their Games Database or community-based online databases.

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.

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... 

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 

ScummVM is a program which allows you to run certain classic graphical adventure and role-playing games, provided you already have their data files. The clever part is by replacing the executables shipped with the games, allowing you to play them on systems for which they were never designed. ScummVM is a complete rewrite of these games executables and is not an emulator per-say. 

ScummVM supports an extensive library of over 325 adventure games That said, you can find a full list with details on which games are supported and how well on the compatibility page.

SDL 2.0.x (or SDL 1.2.x) is a required Library.