pitch shifters

[justforloops@free.fr]

hello, 
I have seen some of you mention "pitch shifters" in their rig .
would you users please mention some brands/models I could look for and/or 
try;

I am looking for something I could use to shift (transpose up or down ...)
my loops while they are playing, preferably foot controlled;

I used to play with synths where the keyboard could be used to 
transpose a sequence while it was being played; 
so having a sequence/loop being played, pressing the E key
on the keyboard would on the fly transpose the sequence up 5 half tones;
releasing it would let the loop come back to its original pitch.

Is there such thing somewhere ? 

Thanks in advance for your help

JFL