Digitech RPs have a very nice single-line pitch shifter, and a very nice rotary speaker effect too. If you could use them both at once, which you can’t do on a single RP, you’d get some decent simple organ tones. (You can actually get decent organ tones with a rotary speaker alone, but a pitch shifter helps a lot.) But if you really want to emulate the sound of a Hammond organ, you need multi-timbral pitch shifting–the kind you get with an ElectroHarmonix POG or HOG, where you have multiple independent pitch-shifted lines running in parallel. Unfortunately, a HOG or POG costs $300 and up, and it takes up a pretty big chunk of space at your feet, too.
I decided last week to check out a promising alternative: the Digitech iStomp, an interesting device that’s essentially a reconfigurable stompbox. I bought my iStomp from guitarcenter.com used for about $60 shipped, a savings of close to 50% compared to buying new.