Simply by using the URL.
[ame="http://youtube.com/watch?v=ZOxkQHbspi8&feature=bz303"]YouTube - Homer Simpson: Picture a Day for 39 Years[/ame]
[ame="http://myspacetv.com/index.cfm?fuseaction=vids.individual&videoid=23471 086"]MySpaceTV Videos: "Blade Runner: The Final Cut by Blade Runner[/ame]
[ame="http://video.google.co.uk/videoplay?docid=4776181634656145640"]Amazing Juggling Finale[/ame]
As you can see, you can use sites like YouTube (of course), Google, MySpace and maybe a few more (I haven't tested many more.) I do have to apologize though as I attempted to convert the old tags ([media]) and error came up in the process, I really do not want to tax the server too much so I am letting it be for now. I just wanted to let you guys know that to post media like this, just use the URL and it should post automatically (if supported.)