I am big into maximizing credit card rewards. One complaint I hear from my friends who only use one card, which is always one of those terrible cards that only gives 1% on everything, is that they could never be able to remember which card to use when.
Walla.by (and Glyph and Smorecard) is an app for the iPhone that does that for you. All three apps are free. For example, if you are in Starbucks, Walla.by would tell you to use your Chase Freedom (5% cash back in Starbucks for the first quarter of 2013) as well as two additional cards in case your Chase Freedom is maxed out, at home, etc.
As a disclaimer, I am not affiliated with the three apps I listed. I just wanted to spread the word