If you use Evernote on the desktop, then grabbing the Android app is an essential step for keeping your notes and files in sync. It also makes for an excellent collaboration tool with the ability to share notebooks with a group. The app is free; $45 annually gives you a premium subscription with additional backup space and faster uploads to the Evernote servers.
If you fancy yourself a power Evernote user, consider the company's Skitch applicaton, which includes inking support for marking notes, clipped web pages, or maps. Anything created in Skitch is saved back into Evernote.
Evernote (Free, $45/yr optional premium subscription)