Shapes of Needlepoint
From the off I'm going to tell you that this app will set you back a hefty $10.99, but if you consider it a comprehensive e-guide for your iPhone then perhaps you can understand the expense. I personally think that if you love your needlepoint, this pricetag is worth it as it's so much easier than carrying around a hardback book and makes it a million times easier to find exactly what you want.
Now that's out of the way, let me tell you why I think Shapes of Needlepoint is worth the download. It really does have everything you will ever need to know about needlepoint and then some. It's a really nice looking, slick app that is easy to use while still retaining a comprehensive dynamic.
As per its name, this app comes into its own in finding that suitable shape to fit into your design and instructions on how to go about forming the look. You can either search by shape, thread count (depending on what space you've got), or even by thread name (if you have something in mind but don't know how to achieve it). The diagrams are really clear and easy to follow, with instructions on how to carry out each stitch through just a quick tap.
It depends on how often, or how seriously, you take your hobby as to whether you'll consider this a worthwhile app, but you definitely won't find a better needlepoint app.