This is the first time I've been brave enough to cut & modify bricks! I felt a bit guilty to start with, but it ended up being quite theraputic after some long, late nights working on a web design project. And I only sliced myself in the finger once with the scalpel, so that's pretty good going for a beginner.
For this custom, I used a torso, arms & hands from the bronze bad-guy from the new Exo-Force range with the head cut off. The legs are a 2nd pair of arms from the same exo-force minifig, with trimmed snowshoes for the feet. The head is a standard black minifig head with a brown helmet on top (Anakin's podrace helmet style), facing the wrong way and with some trimming to the sides of the 'snout'. Then a custom sticker on the helmet and one on the upper torso which makes it more like HK-47 and also masks the edges where the trimming took place.