Time is a big issue. I work full time. My family is dispersed, so seeing them takes time. And I am committed to working out. So any workout I can do at home in less than half an hour is great for me!
Here's an abdominal workout you can do in 10 minutes at home.
Basically, you set a timer to go off every 30 seconds. Pick 20 exercises. Switch when the timer goes. Non stop.
Some suggestions for picking your 20 exercises:
http://exercise.about.com/cs/abs/l/bldynamicabs.htm
http://www.bodybuilding.com/exercises/finder/lookup/filter/muscle/id/13/muscle/abdominals
http://exercise.about.com/od/abs/ss/abexercises.htm