I've recently chanced upon an article about core workouts from Women's Health, a kind of core workout to TIGHTEN your..... of course Core muscles.
I've only experienced doing the "hanging straight-leg raise" a few couple of times when I first enrolled on a gym but it never made into my regular programming, and I could say that these workouts are really challenging specially the "hanging side crunches". Check out the Coregasm workout and find out for yourself why it was on a Women-specific fitness site.
Oh, and here's also the blogpost that led me to finding these core workouts, just visit and I guarantee that you'll learn something from it, or maybe a lot -- about running and other stuffs that you wouldn't normally find in running blogs. I'm already following it, tell me if you also did or why you wouldn't! :)