DivX Stash is an interesting and free service that lets you collect your favorite YouTube, Hulu, Vimeo and videos from other sources and keep them in a central library. The service is in beta.
You can create custom playlists and watch them on any iOS device with a free app or on your Mac. If you see something you like while on your Mac, stash it, then watch at your leisure on your iPad or iPhone. Or you can reverse the process and watch on your computer.
On the Mac, the system requires a Safari, Firefox or Chrome extension. When it is installed, click on the button, and a link to the video is saved in the cloud. You have the option to add some notes, or save it to one of your playlists.