What are some great abdominal workouts that you can do at home?

I’m always searching for new and better ab workouts that can help me get a more defined stomach. I have an ab routine that I follow religiously consisting of a variety of crunches and other exercises, and have a toned stomach, but what I’m looking for is how to get that really defined six pack. If I’m putting the effort in, I’d really like the workouts I’m doing to be the most effective. Any suggestions?