Perhaps the greatest book published for the Marvel Super Heroes (Classic Marvel D100) RPG is the Ultimate Powers book. This is an entire book with nothing in it buy descriptions of super powers. If any one book from the classic Marvel RPG transcends Marvel and the Marvel RPG itself it is this book. This book is still heralded as the most comprehensive listing of all superpowers.
Let's take teleportation as an example. Seems simple enough - disappear from one spot and reappear in another spot. Not so. You have various methods of teleportation. Do you travel through another dimension, is it a folding of reality, or do you create wormholes? There are also a variety of restrictions and potential limitations. Can you teleport others with you? Can you only teleport other objects? Can you take inorganic matter with you (your clothes, etc.)? Can you teleport to other dimensions or time periods? How far can you teleport? Are there any signs that you teleported (light, sounds, smells, etc.)? How frequently can you teleport?
See how such a simple power can be made unique by fleshing it out completely. So while the Watcher, Cloak, and Nightcrawler all teleport it is complete different power for each of these.
Oddly, there were several powers accidentially left out of this book (nine to be exact). You can find the information at http://www.classicmarvelforever.com/cms/ultimate-addenda.html (Classic Marvel Forever website). Elongation! They left out elongation!
Still, a FANTASTIC gaming resource for any superhero RPG and an ESSENTIAL book for the Marvel RPG.