Evernote is a very helpful app to capture notes, thoughts, ideas, images, and audio. I use it regularly as my "digital Moleskine" notebook for just about everything. One thing I've found it helpful for is the ability to take a picture of a page in a paper book I want to remember. Once the app syncs with Evernote's servers it processes the image with OCR technology. Then on a desktop app (not in iOS) you can search for a word in the image of the page. I find this helpful when reading library books. The premium account adds additional features for quicker processing and ability to copy the text.
You are able to use Evernote online, Windows app, OS X app, iOS app, or Android app. Evernote requires 10.6.6 (Snow Leopard) on OS X and that might be a concern for some.