Frequency Shifter will shift all the frequencies in the input signal up or down by a certain amount. This kind of shifting will ruin the harmonic content of the input signal, making it sound dissonant:
Frequency Shifter shines on it's own, but it was primarily created to power up the different "Snapin Hosts" Kilohearts offer. These are bigger effects units that utilize all the available snapins and give you a really fun and creative workflow for combining them in any way you can think of. So have a look at Snap Heap and Multipass right now.
Frequency Shifter is also available as a Rack Extension for Reason over at the Propellerhead shop.