QuiltySolid - A color library app by Steph Skardal with a number of color related tools: color list, color randomizer, color palettes, color palettes from photos, color harmonies, photo eyedropper tool, and color wheel visualization.

PreQuilt - A quilt color visualization tool.  Many quilt artists have uploaded their patterns and you can play with different colorways.  With the paid version you can upload your own designs.

QuiltInk -- Another coloring tool to help plan your quilts that runs in your web browser.  Lots of designer patterns, too!

QuiltingCalc -- A "quilter's little helper" app by Robert Kaufman Fabrics which is a collection of essential calculators.

Villa Rosa Designs - An app from Pat Fryer, a past guest speaker, with the full RoseCards catalog.  You can save designs you like to a personal list.  Search for "Villa Rosa Designs" in the Apple Store or Google Play store.