(Remember: I'm not a doctor, so all standard disclaimers apply. Proceed at your own risk!)
If any form of exercise can be credited for the modern fitness craze, it would be aerobics. In 1968, an Air Force doctor named Kenneth Cooper published a book simply called Aerobics. It outlined his pioneering efforts to measure how much exercise AF personnel needed to pass their fitness tests. The result was a system that assigned point values to exercises based on the amount of oxygen a person's body used. (Aerobic means "with oxygen", as opposed to anaerobic, "without oxygen".)
Why was this such a big deal? Because Cooper was able to determine that AF personnel could pass their fitness tests if they got 30 points' worth of exercise each week... and the point system allowed them to determine exactly how much exercise they needed to get it. For example, an 8-minute mile is worth 5 points; do it 6 times a week, and you've made your 30 points. But you could also run 1.5 miles in 12 minutes, worth 7.5 points; run it 4 times a week, and that gives you 30 points too. In fact, Cooper used the resources of the Air Force to develop point ratings for virtually any sport you might want to try, thus giving any AF recruit the ability to get in shape... and know they were in shape.
The point system caught on quickly with the general public. In 1972 Cooper published The New Aerobics, a book aimed specifically at the weekend athlete, and the rest is history. In fact, the currently-popular 10K steps program is based on his point system.
Points are determined by both the length of time you exercise and the distance you cover in that time. For exercises like rope jumping, where you don't cover a distance, intensity is used instead; for example, 70-90 jumps per minute (jpm) don't get as many points as 90-110 jpm or 110-130 jpm.
You can find all kinds of aerobics info at cooperaerobics.com. In fact, you can download a free PDF of the points chart here. You may not care about passing the Air Force fitness tests, which are being altered somewhat starting in 2010 to include requirements for pushups, situps, and body composition, but the points system can still be useful to you. You can find out what the point ratings are for your current workout routine, and use the charts to help you find comparable workout levels in other exercises. This is useful, for example, if the weather forces you to change your workout. I think this may be the greatest use of the charts, because it can be hard to gauge effort levels between two different exercises.
Say you currently walk as part of your exercise program, and the charts put that workout at 3 points. It's too cold to walk, but you have an indoor bike. You can check the charts to find out how much indoor bike work is equal to 3 points. Because your muscles aren't used to the bike, you might not be able to do 3 points' worth of exercise on it, but you've automatically determined the most you could likely do. You're less likely to hurt yourself now.
The most popular aerobic exercises are walking, running, cycling, and swimming. However, the charts cover a lot of team sports, as well as things like rowing, stationary cycling, rope jumping, stair running, etc. Take some time to look through the charts; you may find an exercise you hadn't thought about, one that you'll enjoy doing.
And in case you're interested, walking 18 holes of golf counts as 3 points.