At a Glance
An emulator of note for the NES--and a handful of other older consoles, including every incarnation of the GameBoy and PC-Engine, a.k.a. Turbografx--is Mednafen. Mednafen is a multiplatform emulation frontend that brings some of the most efficient bits and bobs of other emulators (including, for the NES, FCEUX) under a single roof. While I appreciate the simple fact that it takes the place of several shortcuts on my already-crowded desktop, its cleverest feature is almost certainly the ability to rewind any game you're playing at the touch of a button.
Yes, that's rewind. Just like in Prince of Persia. Unlike Prince of Persia, which accomplished this impressive feat by constantly remembering the past ten seconds' worth of information about the game in progress, Mednafen simply stores the last several intervals of the emulated console's RAM exactly as though it were creating a save state every half-second. While this wouldn't be feasible with modern games (the Xbox 360, for example, has 512MB of RAM, which couldn't quite be saved on-the-fly to even the fastest solid-state drive), the NES, if you can believe it, only has about 4KB of memory, making the whole process as trivial as it is ingenious.