There are plenty of tools for converting YouTube videos, and VLC Media Player is one of them. With its latest update, though, VLC can also simply play YouTube videos when you paste in a YouTube URL.
It wasn't promoted heavily, if at all, at the time of the 1.1 release, but there it is: hit File, choose "Open Network Stream", and paste in the main YouTube URL for a video (just the URL and first parameter, like youtube.com/watch?v=GQe3JxJHpxQ). The process is demonstrated in this YouTube clip, dug up by reader dariusvainilaviius:
Neat stuff, and perfect for those who have bad luck with browsers crashing, or would like a little more VLC-like control over their streaming video playback. Thanks for the tip, dariusvainilaviius! Note: the video example I used in the screenshot, and provided as a URL example, is NSFW.