VisualBoyAdvance: The best one for all GameBoy generations from Classic/Color to Advance, Runs on Windows. Project 64 1.6: Pretty dated, But works nicely for emulating Nintendo 64 games on Windows.
PCSX-Reloaded: A great Sony Playstation 1 Emulator for modern Windows and Mac OS X systems. Fusion: A Sega Genesis emulator that works great on older computers with Windows 9x.
I know some of these have been mentioned here before, But I'll go ahead and list the emulators I mainly use here. I remember having to press a certain key just to go back to the ROM loading window.
When I used to do emulation gaming years ago, I used the following: