I second Jillian michaels. I like ripped in 30 and 30 day shred. Its only 20 min workouts,so doable for a new mom.
I would also take the baby for walks all the time.
Also tabata workouts. Itunes has a cd you can download that you can do with your ipod.
this what really pushed me through the fitness plateau after having my son. Now I teach tabata in my muscle classes. it works. Good luck!