I get bored easily so I'm always on the look out for new routines, especially stuff I can do at home in a small space (usually my bedroom!), in a short amount of time, and that doesn't require any equipment!
This morning I found this : http://www.bellaonline.com/articles/art6530.asp
I just tried it out, modified a little - I decided to make it a cardio work out too, and added in a 60 second jog on the spot between each ab exercise (which certainly got me sweating!). I also altered the "leaning on your hands" exercises because of my wrist. And I can't do planks on my toes, because I am weak... but I am trying! Also, I couldn't do the fit ball exercise because my ball is too big for my little bedroom, but I used a pillow between my legs instead, just as good!
Took me about 20 minutes all up, so I added in some extra jogging, lunges and squats to make a 30 minute routine.
I found it quite good, and my stomach muscles are certainly hurting now!
I think it's a pretty good routine if you want to focus on this area. Dee also had a great suggestion that you could do each exercise in a 5 minute ad break while you were watching TV, which is really good!
Oh and for those of us with lower back problems, abdominal exercises are VITAL for keeping your core strength up and protecting your lower back!