| UAE | The original Unix Amiga Emulator by Bernd Schmidt that all modern Amiga emulators descend from. | downloadable |
| UltraHLE | Historic first Nintendo 64 emulator to run commercial games at full speed. | downloadable |
| Unreal Speccy Portable | Portable cross-platform derivative of UnrealSpeccy running on desktop, mobile and the web. | both |
| UnrealSpeccy | Accurate Windows ZX Spectrum and Pentagon clone emulator by SMT, Alone Coder and Deathsoft. | downloadable |
| Uxn | Browser-based JavaScript Uxn stack machine with the Varvara device layer, with a built-in opcode-stepping debugger. | online |
| Uxn32 | Windows Varvara emulator for Uxn with a built-in debugger. | downloadable |
| Uxn5 | Browser-based JavaScript and WebAssembly Varvara emulator for Uxn. | online |
| uQLx | Open-source Sinclair QL emulator for Unix-like systems. | downloadable |
| uxn11 | Lightweight X11-based Varvara emulator for the Uxn virtual machine. | downloadable |
| uxnemu | Reference SDL2 emulator for the Uxn stack machine and Varvara computer. | downloadable |