For grabbing flash videos, there's a great plugin for Firefox called, appropriately enough, "Download Flash and Video." It doesn't matter what site you're on, if it's a flash file, it'll grab it. It doesn't work with Hulu or the official channels on Youtube (you know, the part that's competing with Hulu), but it works with just about everything else.
(Full disclosure, I use it to "device shift" videos, mainly from my laptop to my homebrew enabled Wii. I would just connect the laptop directly to the TV, but it's a standard def TV, so it doesn't have an HDMI or VGA input. That's why I tried downloading official youtube videos. The system they use to prevent you from downloading them like that also prevents you from viewing them directly through an app, like the official Youtube app for Android, or the various homebrew media players for the wii. I know about Hulu because I've checked for the hell of it.)
Edit: I forgot to mention, that's just for downloading, but I don't need to convert the videos, so it's not a big deal. What modern media player can't handle an FLV file?