There are so many tools out there but I always start with the question of why? If I can't articulate that a tool will make a change to my students' learning, then I would have difficulty in justifying its use. It is simply just a 'cool tool' or the latest new 'must have' tool, but if it doesn't impact student learning in a positive manner, I won't use it.
I start with the SAMR model of adapting technology for learning and teaching - to see how it fits and how it can be adapted through the levels. If I'm using it to just replace something else, for example using Word to replace writing in a book, that's okay but it needs to do more, it needs to reach the 'Redefinition' stage - to create something new that may not have been possible previously to having this tool. Kathy Schrock has fantastic resources on the SAMR model and all other things tech. I've been folowing her since about 1998 and can't recommend her more highly.
One tool I really want to explore is actually a suite of tools - Google Apps for Education and Hapara and now Google Classroom!
Check out the presentations below and see why these tools can make such a powerful change in learning and teaching.