1-UP has a feature that covers some of the history of emulating classic game consoles on modern PCs.  The article covers emulation's gray/black sheep status, all the way from unauthorized software to Yuji Naka's Nintendo Entertainment System emulator for the Sega Genesis to commercial offerings and beyond.  Some companies are even beginning to embrace emulation of old titles, such as Sega's Sega Smash Pack compilations for PC, Dreamcast, and Game Boy Advance.  Even Nintendo has emulation plans for the next generation of consoles.  Disappointingly, I saw no mention of the Classic NES series or the various other official emulated offers currently on store shelves.  Enjoy those old games and remember to always emulate legally.