Yoga is great for running (though I confess to doing far less of it than I should). In fact, Sage Rountree has a DVD, "The Athlete's Guide to Yoga" (and, I believe, an accompanying book), that has lots of exercises that are helpful for runners.

As far as core exercises go, I like planks, side planks, and hip-raises. Push-ups are surprisingly good for core (with a little upper-body thrown in), and crunches work nicely. For extra "fun," do them while draped over a balance ball.

I'm sure folks will chime in with other suggestions.

Don

You might want to try Running Times Magazine and search for the strength videos ( they had something in the last magazine that showed a few and directed you to the website to do others)

I used the video a couple of times and the exercises were pretty good.  I incorporated some of them into my routine.  You can follow their program - but the videos move quickly and I was unable to keep up doing 10 when the person on the screen showed doing 5 before they moved on.  But, for some good exercises - these do pretty good.

I have a large exercise ball and that can help strengthen your core!

Reverse crunches are good too. I didn't see anyone mention them, though they mentioned all the rest I thought of. Also dancing can help. I like to occassionally pop in a dance workout video and they almost always do lots of ab-ness. lol.