Yes. there's a plug-in for Firefox which allows you to download You tube Videos.
The name escapes me at the moment but I'll find it out and get back to you. it's very popular on the firefox plug-ins page so it should be easy enough to find.
Edit
as I said.. easy to find
http://javimoya.com/blog/youtube_en.php
saves in .FLV format. The problem then lies in finding a decent (ie free) FLV player/ converter.
I have one of those too.. again, I'll get back to you when I find out
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