I use http://www.7-min.com/ as the bare minimum workout on days when there's not time or I'm too lazy to do anything else.

The most important thing I've found in getting more fit is to consistently do something every day. Getting started is the hardest part. Having something that only takes 7 minutes, and requires no more equipment than a chair, takes away a lot of excuses and makes that bare minimum easier. Once that's a habit, throwing in longer workouts isn't so hard.