What 'Tech Tool' Has Had The Greatest Impact On Your Teaching?

I am wondering what tech tools have had the greatest impact on your teaching and student learning. What has really transformed how you go about the teaching/learning process?

By tech tools, I mean: gadgets, smartboards, web2.0 tools, browser plug-ins, software, etc....a very broad definition.

I think it would be handy to create a list here of what people really use on a day to day basis. Of particular interest would be tools that were not used in the recent past but have now become a "must have" :)