Please help me identify this blue BIONICLE figure.
The set in question is 8562 Gahlok. It's missing the legs and parts of its head.
Every LEGO brick has an ID printed somewhere on it. Grab the ID from any piece and then head over to http://brickset.com, then do a search (top right) and select "parts" from the dropdown list. This will show you all the sets that part is used in and you can go from there.