Nintendo Entertainent Sysetm Emulators
This is the best NES emulator for Windows or OS X. It includes controller support (including the lightgun), online netplay, and support for the Famicom Disk System.
Click to Download Version 1.4.0 (989k)
(LAST UPDATED: June 11, 2008)
Click to Download Version 1.4.1 (2.2MB)
(LAST UPDATED: September 27, 2008)
Official Windows Site - News, Downloads, Forum, Tracker, Links, Release Log
Official Mac Site - Download, Features, Screenshot

This isn't the best NES emulator anymore, but it was the main NES emulator for years without any problems, and it is very user friendly. This verison is for DOS and Windows 95+.
Click to Download Version 0.42 (329k)
(LAST UPDATED: September 22, 1997)
Official Site - Screenshots, Release Log, Patch Archive, Save Game Archive

No screenshots are available.