The BOSS RT-20 Rotary Ensemble Twin Pedal is a simulation of rotary Leslie-type speakers of the type popularized by organists. Not only does it let you adjust almost every aspect of the speaker's performance, it also introduces features indispensable to guitarists.
Four different effect modes combine various speaker models with varying distortion. You can vary the speed of the horn and the bass speaker independently. An expression pedal input gives you real-time control of parameters while you play (for an authentic Gregg Allman sound), while the Brake feature lets you momentarily stop the speaker's rotation. And the rotating graphic display tells you how you've got the speakers set before you start playing.
The RT-20 is a comprehensive speaker simulator with features that make it useful for guitarists, keyboardists, and many others.