Years ago I had a Fender/Leslie Model 16 unit that had a rotating baffle in front of a speaker.
It wasn't a great Leslie sound but in a small 4 piece band it did help us get away from having
the same old sound all night long. You can search all over the internet and find lot of effects
unit that purport to to capture the real Leslie sound. I settled on the line 6 Roto-Machine
pictured at the right. It has a switch to select between 3 different Leslie models or
"filters". It also has continuously adjustable speed for the Fast and Slow modes and adjustable
ramp speed. It can be run with mono or stereo inputs and outputs. The Drive control is supposed
to emulate the drive input to the vacuum tube circuit that the old tube powered Leslies had. The Blend
control adjusts the mix of drum to horn sounds. I use the Danelectro zero-hum D-A-1 Ac adapter on
all my portable effects units. It has absolutely no noise and goes for about $10 on ebay.
Buy a splitter and you can run more than one effect unit with one D-A-1.
How well does this thing work? Well, you can listen to the sound samples below for one answer. For me it easily tops the Leslie Model 16 and of course is much more portable. I make no representation of how this compares to any other similar unit (see list below). I will say I intend to keep it and I have used it on a few recordings with great results. Check ebay or Musician's Friend for reviews and prices. More reviews are at Harmony Central.
Roto is a sample of sounds recorded in stereo with the Line 6 ToneCore
Roto-Machine. Left channel is Steel King and Right channel is Fender Hot Rod Deluxe. This 2.5 minute recording has three
sections (you will hear a short gap between each) and uses the 3 example settings in order from the Roto-machine manual for
the 145, L16 and 122 filters, respectively. I switch from slow to fast so you can hear the ramping effect.
"Roto" too abstract for you? Listen to how it sounds with a track on a good old country song, "Walk Through This World With Me". Included here is the intro and ending with the Roto-Machine contrasted with part of the middle of the song without the effect. The exact settings I used on this song are shown on the Roto-Machine front panel pic below.