I agree with Designing Great Beers, and I use Beer Alchemy on the mac, not Beersmith. If you are into Belgians, Brew Like a Monk can be a source of recipe inspiration as well.
This works well for brewing more traditional styles. You can also take a recipe (from a book, from here, where ever), and tweak one thing, see how it works out. I've made a few recipes by taking the malt bill from one and the hop schedule from another, or subbing out one specialty malt for another.