Easiest is with Firefox and search for "Video Downloadhelper" in their catalog of extension/plugin.
Really easy.

It has some troubles with the sites listed, Flash and Video Downloader add-on usually does the job on Firefox though

Ah, but that was then and I have never found that freeware again. Damn.

So instead, there is jdownloader ( visit http://jdownloader.org/ ), which does v nicely. TOO nicely, in fact. If a site has lo and hi rez options, it downloads them all, including m4a and all manner of unwanted crap. There may be a way that intelligent users can tell the program to save only One format, but short of clicking on each item in a multiple download and unchecking all but the quality and format wanted, I don't know what it is.

Gripe, gripe, gripe. POINT is that the program will save pornhub and other specialty sites so that you can watch and fap at leisure without being interrupted at a crucial moment.

There is also some kind of youtubedownloader freeware that used to work for at least some of my needs, but either they changed it or I've only been able to find something that vaguely sounds like what I used to have going for me because the last time I tried to install that, whatever it was, it was so busy installing a new browser bar and search engine and bullsht that I tediously undeleted the entire thing, some of it by manual search and delete, until I could take control of my own machine again. Hateful.

I know enough not to click through on anything to install standard automatically (recommended– They say), but still, stuff happens. Wish I could find the old dl tool that allowed simultaneous multiple site searches as well as simple snatching, but until that happens, you might find the program I'm recommending to your attention worth checking out.