I bought a number of workout DVDs and did it for a while, but then it fizzled out for me... just wasn't disciplined enough to set a time aside to make aerobics part of my daily routine.
What I finally ended up using was the treadmill... it didn't take a lot of time, it wasn't too strenuous as long as I walked on it and didn't jog or anything (at least for a while post-pregnancy, that changed when it was safe to do so) and it worked out perfectly for me.
What's your favorite way to get in shape after a pregnancy?