Dan Bricklin's® Note Taker HD App (a Software Garden® product) is an app for the Apple iPad.
You can use Note Taker HD to take notes, sketch diagrams, quickly and easily jot down names, phone numbers, to-do items, directions, and more. Pages are flexibly organized by date/time last modified, tags, favorites, and more, with thumbnail images of part of each page to help you find them later. You can also email one or more pages as a single PDF file.
Note Taker HD lets you create pages by writing on the screen with your finger. You can either write directly on the page for large drawings, or have the "ink" you write shrunk down. You can write in large letters on the screen rather than trying to make tiny motions like a pencil. Note Taker HD automatically shrinks your writing so you can fit a lot of text on a page. You just keep writing and Note Taker HD automatically adds new writing next to the old. To quickly correct mistakes, it has a multi-level undo button as well as an eraser -- just drag your finger over the page to erase the "ink" under it. You can read the reduced-size ink as if you had written those letters with a very fine pen.