Google Drive is Google’s file syncing service and office productivity suite. This app is only the file syncing part, although you may have to have it installed for the productivity apps to work (I’m not sure).
Google Drive’s word processor
Google Drive’s spreadsheet application
Google Drive’s presentation application (think PowerPoint)
Keep is a note-taking app somehow associated with Google Drive, although your notes don’t show up in Google Drive. It works pretty well, and also includes reminders, although I prefer using reminders from Inbox (mentioned above under Email)
Dropbox is a really great file syncing service. It has applications for Windows, Linux, Max OS X, Android, iOS, Blackberry. It also has really great third-party integration. You can integrate other services into Dropbox, and do other cool things with your files.
I use it a lot less for file syncing now, though, because it doesn’t encrypt files end-to-end. It encrypts my files during transfer, but their stored on Dropbox’s servers unencrypted. So, if someone were to hack Dropbox’s servers, or an law enforcement agent were to show up at their server rooms with a subpoena, my files would be readable by them. For that reason, I don’t store anything sensitive in there.
MEGA is like Dropbox, and supports a bunch of different platforms, but unlike Dropbox, my files are encrypted on their servers, and they don’t keep decryption keys. So, even if they wanted to, they couldn’t read my files. I like that a lot. But it doesn’t integrate with a bunch of third-party services like Dropbox, so it’s really just a file syncing service.
Evernote is a great note-taking service. You can use it to clip bits of webpages to any number of notebooks for you to keep for later. For example, I use it while browsing the web to save recipes that I find, and then I can pull the recipe up on my tablet while cooking (not something I do much, actually).
Pushbullet is a very neat service that provides you with ways to get notifications from other services which are synced to all your devices (including your computer). They also recently made it possible to send text messages from your phone, using your computer, i.e., you type the message on your computer in the browser pushbullet app, and it sends the text to your phone, which then sends the message. It also integrates well with IFTTT (covered in Automation).