Evernote has long topped my list of apps. I use it every day on my one or more devices (iphone, ipad, computer, Android...). Online and offline access, photo and sound integration, instant upload and OCR are among the great features.
You really can capture anything, access anywhere and find things fact.
Above all, use Evernote to go paperless! Here is a blog post on the topic to get you started. Enjoy!