The following is an Ab Circuit that I picked up about a year ago. I like it because it's quick, simple, can be done anywhere and most important you'll see results. If your just starting out I would recommend doing each exercise for 20 -30 seconds. As you progress and adapt increase 5 - 10 seconds every three to four weeks. Focus on full range of motion and go slow.
Finish of the circuit with another set of the Navy Seals