Core Workout Routine?

Looking for some guidance.... I am approaching the end of my current routine -from the Men's Health 2006 Training Guide, the muscle/strength workout (was supposed to be 4 weeks, stretched to 8 due to injury layoffs), Well I am in search of a new routine (will probably do some combinations of the poster series workouts), BUT I would very much like to include some core work, any suggestions for an online (read:"Free") source of core exercises? I work out in a gym and they have swiss balls and a bosu ball on hand. Money is tight right now, but the Men's Health Core Workout Book(s) are definitely in my future... ..holla at me... japii