Silver Age Sentinels is a superhero Role-playing Game by Guardians of Order, and it's terrific. It is a point buy, effect based system, where you spend point to get generic effects you can define for yourself, much like Champions, my first Supers Role-playing Game. Unlike Champions, character generation is quick and math light, while still giving you tremendous versatility. I can't think of a single hero I couldn't reproduce in SAS, and many of the classics already have some analogs in the setting.
Also unlike Champions, it's easy to build really amazingly powerful characters at game start. You can't necessarily pull out Superman, but you CAN make someone with parts of his powers. For example, you can give someone Kryptonian level strength. There are three stats in the original version of the game, (They produced a D20 version also, which I never bothered with.) Body, Mind and Soul. There are also a wide array of powers to buy, and advantages to represent those characters built on training. Since point costs are simple numbers character generation takes little time once you read the powers. Game play is smooth and straightforward.
Silver Age Sentinels would work fine as a Play By Post game.