24 volts is probably used more often than 28 volts and quite possibly 72 volts is more common than 60 volts. 15 volts is also commonly used.

I've been using 72V extensively and found that 60V was more common simply because it's in the "Low Voltage" category. It was battery-powered, though. Still, why 3.3V and not 3V? Most(every?) of the IC's accepting 3.3V could accept 3V so I assumed there is some historic reason behind the 3.3 value.

We could also add 1.8V and 2.5V to the question.

It's a number somebody picked from a whole set of tradeoffs. Note that 3.3 V is just about 2/3 of 5 V.

Everyone's right in one way. There is no way to close the question other than deleting it so I will leave it here unanswered.

Actually I think this is a really good question. It's kind of more historical though I guess (and I don't know the answer either).