Emmin ja Terhin treeniblogi: Vatsalihastreeni!!! (T)
The best ab and core workout I have ever done! It´s in Finnish but there are good pictures to explain the movements. You should do each movement from 60-90 seconds (the last one 8 times with your legs up) and have a 30 second recovery.
Fitlandia » Nopea vatsalihastreeni
Fitnessmallin vatsalihastreeni - Sara Nikulainen - YouTube