I don't know how low-impact it is, but my knees are both shot (right one worse than left) and I really enjoy the Hip Hop Abs series. The time flies by, you learn different dance moves, laugh at the funny trainer guy because he's so proud of his abs and shows them off all the time, and then the next day your abs hurt so bad you can't even laugh. (Good hurt, though.)
I can't even do lunges, my knees are so bad, but I can do Hip Hop Abs. I also like the Carmen Electra strip-tease series that she has out (I found them free on-demand on cable.) That's like NO impact, but it works your muscles and tones them.
Of course I'd never do either of those series without knowing that the boys were out of the house for a long period of time. I'm not really the strip-tease type and hip hop isn't the best style of dancing for me..... actually my style is more like Elaine on Seinfeld.
July SW: 129