i was aware of these websites for sometime but never had one work for me. there's several but here's one that i know works on my mac; media convert.
basically you paste a url from google video, youtube or whatnot into it and specify what format you want to have it in. could be for your mobile phone, psp, ipod or just to play on your computer. so far the results have worked really well and is only limited by the crapy source videos on the web.
so please email or post any good finds from these sites that are worth converting and keeping.