Phil posted this, but I hadn't seen it before and I think it bears repeating: A heart rate calculator which tells you what your zones are, based on your resting and maximum heart rate. This is helpful for those of us for whom the standard formulas don't work very well.
My calculated zones, just for the record, are below. I find it fairly difficult to get under Zone 4 when I'm running, and hard to get up into it when I'm cycling. Haven't checked my swimming heart rates for a while, they're probably through the roof.
Zone 1, walking: 127-138
Zone 2, jogging: 138-149
Zone 3, running: 149-160
Zone 4, hard: 160-171
Zone 5, all-out: 171-182
Actually, now that I look at the numbers, as a mnemonic device you could probably take the highest heart rate you've ever seen on your heart rate monitor (182 for me), and then simply count down in ten-beat increments from your maximum for the lower limit of each Zone (172, 162, 152, 142, 132 for zones 5, 4, 3, 2, 1) and that would give you a pretty close approximation of your zones, too.