GENS is a very nice Sega Genesis emulator. It is available for Windows. It has a lot of options and capabilities like debugging the various processors in the Genesis.

From the gens.txt file:

    _________________ 
   /################/
  /# INTRODUCTION #/
 /################/
 ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Gens is a Win9x based emulator for the SEGA Genesis/Mega-Drive/Sega-CD/
Mega-CD/32X systems, it allows you to play games developed for these classic
SEGA 16bit game consoles on your x86 compatible PC.

Gens is developed by Stéphane Dallongeville and has been coded 35% in C/C++
and 65% in Assembler. It runs under Windows 95/98/ME/2000/XP and requires
DirectX 7.0 or above to be installed on your system.

The genesis compatibility of Gens is very high :

    * about 93% of games work perfectly or with no noticeable problems.
    * a further 5~6% of games are playable with minor problems.
    * 1~2% of games are unplayable.

(figures quoted are for Genesis/Mega-Drive only from tests conducted by the
 Genesis Project and are approximate.)

The compatibility for Sega-CD/Mega-CD emulation has been estimated as being
about 90% or higher and the 32X system compatibility turn around 75%.

Here is a screen-shot of the program.