2. Google Drive
While Google Drive's ability as an office suite is limiting, the app is an excellent solution for cloud-based storage of anything and everything you encounter.
Drive is useful for storing things like notes, files, and images -- all of which, once uploaded, are universally synced and accessible from any computer or mobile device. Its built-in "scan" feature is particularly useful for productivity: You simply tap a button on the app to capture an image of any physical document; Drive will then extract the text from the image and make it easily searchable for the future.
Google Drive comes with 15GB of free storage space (shared with Gmail and Google+ Photos). You can purchase additional storage starting at $10 a month.