The most easy way to download YouTube Video:
Step 1: Find the video you want to download and wait for it to completely finish loading.
Step 2: Minimize your browser (do NOT close it, some browsers delete the temp directory upon closing), and navigate to the directory /tmp
Step 3: Drag the flash file onto your desktop. Voila!
This definitely works with firefox, and probably most other browsers as well. Enjoy!
Update:
This post is outdated and may not serve the stated purpose.
Sandeep Bhalla's Blog 
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