If you use Microsoft services every day, chances are SkyDrive is already a part of your workflow. The new Office applications save your documents to SkyDrive's servers by default. You can store photos, videos, and more there, too. From a SkyDrive app, you can launch the Office Web Apps in a browser to work on projects away from Word, Excel, or PowerPoint on your PC. Pay $10 to add 25GB, $25 for 50GB, or $50 for 100GB per year to your free 7GB of storage. SkyDrive and Office are also a powerful combination if you use a Windows phone.
Google Drive is an essential cloud storage app if you use Google's productivity services, including Gmail, Docs, and Spreadsheets. You can not only store and sync important files across devices, but you can also create documents and spreadsheets from your tablet. Plus, you can share files easily with team members, who can edit the same document with you at the same time. No more passing printed drafts back and forth. On Windows 8? Access Google Drive in a browser.
If you just need to keep personal and professional documents at your fingertips, Dropbox eliminates the need for a filing cabinet. It doesn't come with extra services (such as the Office Web Apps, the Google Docs suite, or Evernote's note-taking features), but it's dead-simple for uploading and syncing up to 2GB of files across multiple devices for free. Share files and folders with other users, and if you let it sync files and videos automatically from your tablet, you get an extra 3GB. Up that to 100GB for $9.99/month, or check out Dropbox for Teams, starting at $795 for 5 users, offering 1TB per person. If Dropbox loyalists aren't already on your team, check out its worthy competitor, Box.