The Nike+ Training app has a lot of good ideas. I think most of the workouts are a little longer but you can divide them up over the course of the day. It's free to download. Someone on here recommended it to me a few months ago and I really like it

OP here--I'm just getting back from the weekend, and I'm grateful to see these suggestions! These are lots of great suggestions here! I'm determined to use this to do 15 minutes of exercise TODAY instead of making another excuse. Thank you!

Here's the new York times 7 minute workout-- but it is supposed to be done hard/ fast enough to be pretty uncomfortable

http://well.blogs.nytimes.com/2013/05/09/the-scientific-7-minute-workout/?_php=true&_type=blogs&_r=0

Anonymous wrote:You're not going to raise your heart rate enough in a 15 minute work out to burn any fat. 30 minutes is really the minimum.

Go back and pinpoint the part in op's post where she says she wants to exercise for the purpose of burning fat.