Nudibranchs are one of my favorite inverts. After diving in Papau New Guinea, I was amazed by the incredible variety of their colors and forms, and due to their bright tones you can usually spot them from several feet away. This photo gallery has a very nice collection of them from many different regions and gives you a primer on their different bodily shapes. Thanks to Tunze for the tip. LINK
Note: For the most part, nudis are not reef safe or are generally considered a “difficult” animal to keep. Don’t add these to your tank unless you have researched their exact requirements.