As of October 2, 2000, LoopyNES 1999.10.18 is one of the most accurate NES emulators that run on older MS-DOS based PCs.
Like NESticle, LoopyNES runs on DOS and on Windows 95, 98, and ME (but NOT NT, 2K or XP).
Unlike NESticle, LoopyNES has full pointing device support (for Zapper-compatible software), can do mid-screen palette changes, and emulates vblank reading correctly.
It was also one of the first NES emulators to do sprite 0 hit detection and cycle-accurate raster timing (for Pole Position-type games) properly; for this reason, it is the emulator of choice for NES game developers with older PCs and for those who want to play those nearly impossible Nintendo cartridges.
It does have some minor problems with sound however that show up in the percussion in the background music in Dr. Mario and on some NSFs.
As of mid-2002, FCE Ultra is more accurate, but it appears to need a faster PC.