MS Office 365, Google Docs, Google Drive, Gmail, MS Lync, iMessage.

While perhaps not productivity apps per se, the combination of Flipboard and Readability helps me to quickly take the pulse of all the specific content that I need to stay abreast of (Flipboard) and easily save the information I need to read or re-read later (Readability).

Depends on your business but I am a big fan of project management/collaboration tools to help set and track deliverables and milestones. MS Project or Sharepoint, Basecamp, or Asana (which has a free option) are all worth checking out.

I use Dropbox.