If you (like me) enjoy alot of Youtube video's from random people, not seperatly downloadable, this link will help you out:
KeepVid: Download videos from Google, Youtube, iFilm, Putfile, Metacafe, DailyMotion!
Its very simple operatable and works very fast.
Simply copy/paste the correct link, select the correct site, and you get a link to the video from the YouTube page.
However, files are saved in FLV format, and not all media players can play that. The site offers a trial program to switch them to wmv or avi-files.
Another alternative is Media Player Classic. It has most codecs builtin and can also play FLV files. Download here.