The Ubiquitous Amiga Emulator is without a doubt the Amiga emulator of choice. With the classic and wonderful GNU public license idea, UAE has been blessed with ports to all operating systems and has received much nuturing from all of its authors. UAE now includes Picasso '96 support that allows you to change resolutions natively and flexibly.

Fellow is the second Amiga emulator to be released and is only available for MS-DOS. To my knowledge, Fellow is more primitive than UAE, but correct me if I'm wrong.