Calculess will be available soon on iPad. There are a ton of calculators to choose from now, but few I'd buy. I made it through high school, Georgia Tech, and University of Florida with my TI-86. I love having a multi-line display and the ability to see and edit any part of my expression. Calculess makes it easy to insert any previous expression or result onto the current line. An extended history is available from the toolbar, so you can refer to previous calculations or substitute them into the current one. Expressions can be arbitrarily complex.
There's a bit of room left for some more buttons, so I'll be listening to feedback.