Flash. It does crash, has many security flaws and is quite horrible. But nevertheless, it is still widely used. I.e. in flash videos, like on Youtube. But now, you got a choice: To switch to HTML5 Videos. On Youtube.
To do so, goto / jump ( 🙂 ) http://www.youtube.com/html5
There you can activate / deactivate HTML5 and Testdrive the new features.