Downloading YouTube movies is something that may be occasionally required when you need your video to be available offline.
In the paat we wrote about how to download YouTube videos to your iPhone, wrote about how to download YouTube videos to your Windows Mobile smartphone (actually 3 ways: 1, 2, 3) and here's how to do it for free with an Android smartphone:
Meet YouTube TubeMate!
TubeMate downloads YouTube clips as HD video or MP3s straight to your phone.
Of course, this is a completely forbidden thing to do so I definitely do not recommend on using it. Not sure how long this will be around, but if, as an exception, you must do this, just once, and never again, forever, and ever, here's a link to download the application (for free…).