Jillian Michaels' 30-Day Shred :D.
One of my favourite topics on EW but seriously, it's great for people like me who a) lack motivation, b) lack time and c) spend the entire session swearing like a trooper at that skinny b1tch on TV shouting at me to give her some effort.
It takes 30 minutes out of your day and that includes getting changed beforehand and getting a quick shower afterwards. Job done. You can download it on www.exercisetv.tv.