(Peacock) Mantis Shrimp
Posting this guy, because (and because I didn't see any in the tags)! I got one today. Eeeeeee. He's just a little guy, and he just shed so he's hiding in the back of the tank whining to himself. He's only about 3 cm long, so just a baby. They can reach lengths of up to 30cm (12 inches). I think...I will need a bigger tank in a year. =) And probably an acrylic one as well.
After watching the awesome video on the pistol shrimp (of which I have two, an orange one and a clear/green one; I'm having a lot of trouble finding the exact species names for each) I figured this guy is just as cool, if not cooler because they get so much bigger and can actually shatter the glass of an aquarium.
Articles and videos:
http://en.wikipedia.org/wiki/Mantis_shrimp
http://www.youtube.com/results?search_query=mantis+shrimp&search_type
"Both types strike by rapidly unfolding and swinging their raptorial claws at the prey, and are capable of inflicting serious damage on victims significantly greater in size than themselves. In smashers, these two weapons are employed with blinding quickness, with an acceleration of 10,400 g and speeds of 23 m/s from a standing start [5], about the speed and force of a .22 caliber bullet. Because they strike so rapidly, they generate cavitation bubbles between the appendage and the striking surface [5]. The collapse of these cavitation bubbles produce measurable forces on their prey in addition to the instantaneous forces of 1,500 N that are caused by the impact of the appendage against the striking surface, which means that the prey is hit twice by a single strike; first by the claw and then by the collapsing cavitation bubbles that immediately follow [6]. Even if the initial strike misses the prey, the resulting shock wave can be enough to kill or stun the prey.
The snap can also produce sonoluminescence from the collapsing bubble. This will produce a very small amount of light and high temperatures in the range of several thousand Kelvin within the collapsing bubble, although both the light and high temperatures are too weak and short-lived to be detected without advanced scientific equipment. The light emission and temperature increase probably have no biological significance but are rather side-effects of the rapid snapping motion. Pistol shrimp produce this effect in a very similar manner."