Emulators of the iconic Leslie speaker, the innovative, rotating, Doppler Effect projection unit that became married to the Hammond B3, though they can easily be used for other instruments like Electric Pianos.
Note some musicians don't like them at all, let your ear be your guide. Also note some of the Hammond organs recommended on this site come with Leslie speakers built into them.. You can use them, or turn them off and use a separate speaker, or combine them for some unique sounds.
Spinner (NuBi Plus Pack)
Download program
Stunning retro look, dizzying array of options.
Mr Donald (Soundfont Legacy Pack)
Website
Animated interface, lots of options including MIDI-controlled on/off.
iSpinner
Website
Drop-dead gorgeous retro look. Limited options, but works very nicely.