Check out the many species of Mantis Shrimp.
Neogonodactylus wennerae
This one is a Neogonodactylus wennerae. They are the most common species that hitchhikes in on live rock. They change color depending on environment, and can range from bright green to brown to maroon. They only grow to about 3", and are relatively peaceful for a mantis. They can't hit hard enough to break thick-shelled snails or hermits. They are also more active than the chiragra species, and are often out exploring the tank. She loves to eat krill from a bamboo skewer.
Thank you Kelli for this page
This is Odontodactylus havanensis. They are fairly common off the coast of Florida and in the Caribbean. They are in the same family as the peacock mantis, but these only get about 2.5-3". They are one of the more peaceful species, but they like to snack on small hermit crabs. They are definitely the "cutest" mantids with huge buggy eyes. They come from deeper water than most other mantids, 60-100 feet down. These are my most active mantids, and the species I have the most of ( I ordered 3 online and the guy sent 8 of them). They come out and beg for food whenever I am in the room, and they will attack a bamboo skewer even if there is no food on it. My smallest one is only 1/2" long, and it is a struggle for her even to tackle an adult brine shrimp.  The tiny one has taken up residence in an abandoned shell from a small hermit, and uses it as her burrow. 
Odontodactylus Havanensis
Pseudosqilla ciliata
Here is another spearer, Pseudosqilla ciliata, which is a spearer that evolved into a smasher, then back into a spearer. They have the aggressive attitude of a smasher, but are relatively reef-safe (except for fish). They burrow in fine sand, and are out hunting most of the time. They will spend hours stalking small fish. They also change color depending on environment, from yellow to green, green with white stripes, and even black. 
  Lysiosquillina maculata
 These are bit different than the other mantids. Instead of "smashing" type arms, they are spearers, with arms like a praying mantis. They eat fish and shrimp. They come from the south Pacific, and make deep burrows in fine sand. They are the largest mantis shrimp, reaching over 16" and living over 30 years. They live in monogamous pairs, with the male doing most of the hunting. They are another species that almost never leaves its burrow. This species is very uncommon, but I was lucky enough to find a pair. The ones I have are very young and small (for this species, anyway), with the male at about 5" and the female at 6" . I have set up a special tank for them with a cut-away burrow so I can watch their behavior.
It's difficult to see but here is a closeup of the claws.

Odontodactylus scyllarus

The peacock mantis is Odontodactylus scyllarus. They are found in the Indo-Pacific. Peacocks are one of the most colorful and aggressive species of mantis shrimp, and adults are capable of breaking glass. This usually happens when they are burrowing and hit the bottom of the tank, although they have also been known to break the sides of tanks when they are teased through the glass. They grow up to 7 inches, making them one of the largest species of "smashing" type mantids, and one of the largest common species in captivity. They are very interactive and can learn to recognize their keeper.

A few interesting facts about mantis shrimp:

They are actually not true shrimp, but a separate family of crustaceans called Stomatopods.
They have the most advanced eyes of any animal, and have 16 different types of photoreceptors (compared to our 3). They have binocular vision in each eye, so they have excellent depth perception with only one eye. They can see 4 different colors of UV light, and can also see polarized light. Some species, such as Odontodactylus havanensis, actually communicate to each other using signals of polarized light.
Mantids also have the strongest strike of any animal, relative to their size. G. chiragra and large peacocks can hit with a force equal to a .22 caliber bullet. They also have the fastest strike of any animal. Their strike is so fast that they vaporize the water at the point of impact, causing a small implosion. This makes their strike even more destructive, and stuns their prey. Mantis shrimp are also one of the only predators of the blue-ring octopus. They smash the octopus until the venom glands burst, and after the venom dissipates in the water, they eat the octopus.

More mantis Links
Gonodactylus chiragra
This one is a Gonodactylus chiragra, which grows to about 3-4". It is not very big, but it is the most bad-tempered and most hard-hitting mantis there is. A 3" chiragra has more strike power than a 6" peacock. Chiragra and peacocks are the only species that are tough enough to crack a tank, and the chiragra is even more aggressive than a peacock..
Chiragra are very shy and rarely leave their burrows (I have only seen mine out twice in over 6 months). They bash their way through solid rock to create intricate tunnels, with multiple entrances and exits. Her favorite food is turbo snails, which she smashes to pieces. Then she uses the shell fragments as builing blocks for her den.

These shrimp are not easily available and came from several sources years ago