Excellent reddit post on "can't live without" programs/addons. Below is a copy of one of the more detailed entries from redditor microwavedCheetos (sorry, too lazy to summarize)... Most people covered the common software out there, so I'll focus on the less commonly...
open source
“Federate Wiki” ~ Wiki creator Ward Cunningham’s next project
I love wikis and how they work, but most of all I love the philosophy behind wikis. Ward Cunningham, creator of the whole wiki concept and initial implementation, continues to refine what a wiki is and should do with his github project, Federated Wiki. The future for...
Anti-theft solution for laptop, phone & tablet ~ Open Source Prey Project
The Prey Project is an open source anti-theft application for laptops, phones, and tablet devices. When installed and activated, it will allow you take a picture with the devices camera, lock the device, etc. The worry I have is exactly that it provides an easy method...