Called Knit Handy and Crochet Handy, respectively, these apps will let you determine how much yarn you'll need for basic garments and accessories knitted (or crocheted) at a variety of gauges for a wide range of sizes. Crochet Handy also has conversions single, half-double, double, and treble crochet stitches. All of this is available at the iTunes store for just $0.99 each.
For more information and an official press release, go to: http://ow.ly/7Azi8
For Knit Handy, go to: http://ow.ly/7AB9V
For Crochet Handy, go to: http://ow.ly/7ABza