I tried the so called 'Tabata intervals' today for the first time.
The protocol consists of eight 20-second maximal sprints, with a 10-second recovery period after each sprint. So the whole exercise takes only four minutes.
Naturally you should reserve time for a warm up and down in addition to this tough exercise.
Tabata intervals are based on the famous 1996 study lead by Dr. Izumi Tabata in Japan. The study showed that this simple and short exercise is very effective for raising your VO2max.
Also in this blog there was a posting about the same subject recently.