Spiff's randomizer webpage is pretty cool, but the formatting does not behave very well on my phone. So I wrote my own.
http://www.fecundity.com/pmagnus/sentinels/
Here's a run down of its features:
It is designed as a webapp, to display nicely on a smart phone screen. On an iphone, you can add it as an icon on your homescreen and it will run like an app.
It caches the content of the page, so you can use it even when you don't have a network connection.
You can bump specific choices without rerolling the whole scenario. If it gives you a villain you don't have or a hero you've played too much, then tap 'again' to change just that part of the scenario.
Because you can bump specific choices, it doesn't require you to enter which things you want to have included. It knows about everything, and you can nix anything you don't have or don't want.
It includes alternate versions, for heroes and villains that have them. For example, if it randomly selects Baron Blade as the villain than it tosses a coin to decide between original recipe and mad bomber.