Bulk Media Downloader extension is a lightweight alternative to media grabbing extensions like FlashGot or Flash video Downloader extension.
It grabs and downloads all sort of media resources in bulk. You can filter different types to prevent cluttering the grabber window. It also has a pause button to stop appending more resources.
Note that network observer is only active when the grabber window is open, hence this extension has no effect on your browser performance when the grabber window is closed.