Pilaties.

Pilates is an excellent exercise for strength, though it doesn't burn a lot of calories, so I use it as one of the many exercises I do throughout the week. Pilates focuses on strengthening the "core" (your abdomen) and a lot of the exercises focus on that. It's been great for my overall strength and even for flexibility. I don't know that I'd say Ive had results based solely on it independent of others.

The only equipment you need is a mat of some sort (ie: yoga) and a pilates dvd or pilates class to attend. Some variations will include bands, balls, or weights - just depends on which you do.

I used to do Pilates at my gym in addition to a cardio/weights routine and I was very toned with a strong core. Since my baby I've gained so much weight BUT I just started doing Pilates 1 week ago (I sprained my ankle like July 2 and haven't been able to do a lot) and I feel like I walk taller...therefore making me look better. I also feel more of my stomach muscles working just walking throughout the day.
And if you do any rough workouts that leave you super sore, you can do the flexibility Pilates for a GREAT stretch! :]

I say give it a try.

I got the 10 Minute Solution Slim & Sculpt Pilates that comes with the band (I needed the band to recondition my sprained ankle) and I like it a lot. It was only $10! There's 5 different Pilates routines on the dvd and you can create your own workout. I recommend it.