your more than likely going to be breaking the law by performing such an act. But If you must ask.

Your going to want a copy of Youtube Downloader for Youtube flicks and then there is a Phillips App for Go Gear Aria players that will do just about any flash embedded video. Vimeo etc..

For ripping stuff of your dvd's your going to have a more complex time. You can use stuff like VobDec or other apps but I would recommend going to Cnet and doing a search for DVD to Ipod and then trying a few apps and picking the one you like the best. Somefolks like it easy and some like it hard....