M-Tron, a PC and Mac VST 2.0 instrument, offers a library of 28 classic Mellotron sounds and some added features. Obviously, MIDI is now part of the package. Envelope attack and release controls, which the original machine didn't have, have also been added. A toggle switch accesses the envelope controls and the sound selections. Every bit as charmingly lo-fi as a real Mellotron.
Intonation is occasionally questionable, timbre is inconsistent, notes sometimes warble a bit.
In a word, authentic. Rarely heard 'Tron vibes, accordion, mandolin tremolos, and oboe are also included. The rhythm banks are hilariously hokey and probably not all that useful, but in the interest of completeness they're appreciated. M-Tron puts volume, tone, pitch, attack, release, and pan under MIDI control. I think you'll like it.