Now, that's pretty broad, but it can be anything in technology from software, to hardware to basic physics underlying a cool technology (say why there).
One thing that I find is pretty good is the way YouTube only loads enough of a video to buffer a few seconds (maybe tens of seconds) of a video and then lets you watch.
It saves bandwidth for them and you.
Now, it is a double-edged sword, as you can't just load a video then come back to it and seek through the video, but overall, I think it's a great compromise for speed, bandwidth, and viewer experience.