BizHawk is a free open source multi-system video game emulator for Windows, written in C#. BizHawk is designed primarily for the tool-assist community, so it has a lot of diagnostic and scripting features.
It has high-accuracy emulation for the following systems: Apple II, Atari 2600, Atari 7800, ColecoVision, Commodore 64, Famicom, Famicom Disk System, Game Boy, Game Boy Advance, Game Boy Color, Game Gear, Genesis, Intellivision, Lynx, Master System, Neo Geo Pocket, Nintendo 64, Nintendo Entertainment System, PC Engine, PC Engine CD, PlayStation, Saturn, Sega-CD, SG-1000, Super Famicom, Super Game Boy, Super Nintendo Entertainment System, SuperGrafx, TI-83, TurboGrafx-16, Virtual Boy, Wonderswan, Wonderswan Color, and ZX Spectrum
I discovered BizHawk while checking out tool-assist speedruns on TASVideos. After seeing how versatile it was, I downloaded it and started using it. I have since switched over to using it as my primary emulator for most systems. One of the things I like most about it is the common interface for each system. Rather than having to learn the unique way of handles things for a bunch of different emulators, most of the systems use similar configuration screens and tools.