Evernote (Free) is just one part of an excellent, access-from-anywhere note-taking system. In addition to using Evernote on your iOS device, you can also create and get to your notes from a variety of mobile devices (including apps for both the iPhone and iPad) and any Web browser on any computer. A free Evernote account links all your notes together.
Evernote is a mature and popular app, with an impressively streamlined interface that shares similarities across its multiple platforms and gives you many ways to create notes and collections of notes called notebooks. Your notes can be text, images, or Web clippings, but a premium account will let you save other file types, too.
In addition to typing in notes via Evernote (using word-processor-style formatting tools), you can also add images and record audio or iSight notes to attach to your project. You can tag and search all your notes from anywhere (with fairly amazing character recognition in images) and export notes in a variety of ways (once synced to desktop). Plus Evernote syncs up your information as often as you want.
There's a lot to like in Evernote, and the fact that it's free makes this app easy to try out. If you're looking for a note-taking app that syncs across all platforms--or just an easy way to keep track of your digital odds and ends--Evernote is a worthwhile download.