Mesen has two different version, first one is Mesen and second Mesen S. Difference between them:
Mesen supports only the Nintendo Entertainment System (NES) platform and Mesen S supports the Super Nintendo Entertainment System (SNES), Game Boy (GB), Game Boy Color (GBC). Mesen S includes an extensive set of debugging tools.

NES is an 8 bit video game console that was produced and first released by Nintendo in Japan and has a name as the Family Computer, widely known as the Famicom in 1983. Then the NES got a remodelled version, which was released internationally.

Mesen has numerous of nice features as:

-Save states
-Video filters ( Changing texture filters )
-Netplay ( only peer-to-peer by IP address)
-Rewinding ( Like time machine, you can rewinds for example 10 seconds of gameplay )
-Overclocking ( Speed up gameplay )
-Cheat codes
-HD packs