Was just looking for the same thing a couple of weeks ago...and while I found lots of free videos most of them were not ideal because they skipped some of the explanations or only showed one or two drills rather than a complete core session.
The links above are good but the one below is the one I found most useful if you want a good, short, complete core workout without digging through dozens of other videos to piece together a routine.
http://www.strongerrunner.com/media/
there's a link for a free ebook and 10 min video download..you have to enter your email, so that may scare off some people...but this really was the best free core workout video for runners that I found.
Jay Johnson's site does have lots of good videos on though and would recommend that to get some ideas as well.