I found "All video downloader" from the ubuntu software center(12.04 lts Precise Pangolin) a good program which has the following PROS:
Can download from a host of sites such as youtube, DM, Vimeo, etc.
Easy to use with a good UI.
Has batch mode where you can copy-paste many video URLS and will download one at a time.
Supports down load in many formats (FLV, MP4, MP4-HD, etc) but always first downloads in flv first and then converts it to whatever format you had selected.
CONS :
A bit slower( compared to the ones I have in windows), if you want to download in any format other than FLV.
Try Keepvid downloader which is free and safe to use. I used to get loads of videos from web via this tool. Though it sometimes may not handle some clips which depend on the situations.
You can install some Firefox or Chrome plugin for doing that, such as Video Download Helper.
Or, for Youtube an other popular websites, you can use KeepVid, wich works on the browser.
For the second question, I suggest you to create a new one. I hope it works to you!
To convert videos into different format, handbrake is a handy tool.
I found "All video downloader" from the ubuntu software center(12.04 lts Precise Pangolin) a good program which has the following PROS:
CONS :
Try ClipGrab. ClipGrab is a free downloader and converter for YouTube, Vimeo, Dailymotion and many other online video sites.
Try Keepvid downloader which is free and safe to use. I used to get loads of videos from web via this tool. Though it sometimes may not handle some clips which depend on the situations.