Recent versions of RealPlayer allow you to save YouTube video to your harddrive in Flash video format (YouTube's "native language," not the easiest format to work with, and I find conversion of FLV to other video formats often less than satisfactory).

Anywho, if you see something on YouTube yo'd like to keep, it becomes an easy matter to snag it. BUT...if you *really* want to keep it, copy it to another location on your haddrive, or burn it to CD, because many of these video-snagging tools can silently delete the copy you made (in the app's default save folder), should it become legally important for someone to do so.