I can give you some good advice. No need to be blur by so many suggestions.
Most of the advice given by the rest are workable but it depends on what you want...
You mention you need a YT downloader that can download
multiple videos.
Most of the advice given so far are browser dependent and can download one YT video one at a time only.
So, it will not fulfill your need.
Besides, you also may like to download from
other sites beside YT, hence all the advice given thus far cannot do that, except
downloadhelper for FF, which is an FF addon and hence browser dependent.
I've used all the examples given, tested all of them and even more.
If purely for single YT video and browser independent, use the advice given by one of the bros:
www.savevid.com
(You need java applet)
If it's for multiple videos (YT and other sites), then I recommend this:
http://atube-catcher.dsnetwb.com/video/
It also has burning into DVD, CD features, etc.
NOTE: When installing the software, remember to untick all other installs, including the Ask browser helper, which is very difficult to remove if you accidentally installed it.
If you're looking to download RTMP streaming videos, then it's a different solution altogether. If it's for Hulu movies download or NBC copyrighted videos, again it's also another solution that requires some technical tweaking and other software. If interested, then I write about these solutions.