1. Google Drive
2. Video Conferencing
I did not name one such app because I use different ones with different collaborators. Since I am fairly adept at technology, I use whatever others are used to. That is why I use: Adobe Connect, Skype, Zoom, Hangouts, and even Facetime. If I were pressed, I would name Skype as my most commonly used video conferencing app. This is how I connect to co-authors, students, and potential collaborators.
3. Social Media
Social media is my way to learn from people I do not know (or at least know well). My favorites are Twitter and Google Plus. Twitter has a massive reach, and I find many like minds. The downside is the 140 characters limit that collaboration- and I often find myself frustrated by the speed and brevity. Google Plus is a much smaller community, but I often find that interactions are productive and more enduring.
There are many ways that technology complicates our lives, but in collaboration it allows us to collaborate better and further than ever before.