I love the free "7 minutes" app. It has a series of exercises (e.g. planks, push-ups, lunges, squats, jumping jacks) done in 30 second intervals followed by a 10 second break. I use it on my regular 5k lunch-time walks (do it at my half way point), while on the road (in my hotel room) or anytime I want to sneak in a quick work out. I don't always follow the exercises they direct you to do. The 30 second timer followed by a 10 second break is very useful on its own to do the exercises you want to do.
No equipment required!