In yoru browser, AutoChords is as easy to use as loading the web page choosing the style, instrument, key, etc and then you can preview the progression and alternatives too. You can of course use this service in most web browsers for free.
And for iPhone and iPad there's an app. And it's free too! The creator feel that "Autochords is the ultimate songwriting cheat sheet." And much like on Mac or PC, you simply pick a progression style and a key that you're comfortable playing in, and Autochords will show you what to play for your Main Progression. It also suggests "Alternative progressions" that you can use for other sections of the song, like a bridge or backing for a solo.
Thanks to @ReaktorPlayer on Twitter for the heads up on this awesome app.