Saving online videos (e.g. YouTube) and moving them to iPad for later offline viewing?
Safari and Chrome
I had no luck getting any extensions working. YouTube intercepts right clicks on video and shows a youtube context menu instead of the native context menu and thereby blocking the download button.
Firefox
Download YouTube Videos as MP4 and FLV 1.3.8 is an extensions which is compatible with Firefox 11 (as of march 2012). It shows a download button below the Youtube video area.

The download format is .mp4 which is fine to use on an iPad.
Still, if you want to convert the downloaded video for your mobile devices, you can use a simple tool like the MiroVideoConverter, which is available in the Mac App Store for free.