I use iBrewmaster. Kind of pricey, but I love it. It has a lot of Northern Brewer recipes already loaded for free, and there others you can purchase access to. That's the only thing that stinks. It's real handy in putting recipes together. You plug in the ingredients and the hop schedule and it will calculate the IBU's, ABV, OG, and estimate the color for you. It even tells you how your beer will compare to the BJCP standards. You tell it how you want to mash and sparge and it will calculate your strike water temps and volumes for you. It has a handy timer for the boil that gives you an alert when it's time to add something. And I get notifications when it's time to dry hop or move a beer into secondary or bottle it. It's a little clunky until you figure out everything it can do and how to enter data, but overall it's a great app.
I've been thinking about using Beersmith, but I like the flexibility of having my phone out in the garage with me while I'm brewing. If I want to change something on the fly on brew day I can plug it into the app and instantly see what it's going to do.