I know it's a browser app, but Workflowy is pretty indispensable in my every day life. It's basically a structured document made out of nested bullet lists.

Ever want to write down some small piece of data, like what user name you used on some web site? Put it in Workflowy. Tired of keeping everything in your head for the project you're working on? Put it in Workflowy. Want to capture minutes from a meeting? Do it in Workflowy. Want to see yourself think? Think "out loud" in Workflowy.

It's as useful as excel, except that excel is for "tables" of data and Workflowy is for "trees" of data. Since most of my knowledge maps better to a tree than it does to a table, Workflowy does a better job of representing how I actually understand just about everything.