Originally Posted by mr dave

you're basically asking for a device that digitally re-creates a device that was originally created to re-create instrument sounds with tapes (analog). it's one notch too far in traditional synthesis.

it's not like there's a specific 'mellotron' sound, it was just the name of the device that used tapes to determine the type of sounds manipulated by its keyboard. like FD said, the closest modern device is a sampler.