This winter I've been doing the workout at the following link and am seeing good results with my core strength and endurance:

http://www.bicycling.com/training-nu.../core?page=0,1

Heh, heh...the first time I tried this I was stunned at how weak some of these core muscle groups were. There is one exercise where you are supposed to hold a particular pose for 60 seconds. I think I barely managed 15 seconds the first time I tried it.