(FORTUNE Magazine) -- NEVER MIX BUSINESS WITH PLEASURE. It is one of the cardinal rules of the workplace. And in the context in which it's usually offered (office Christmas party + one drink too many + mistletoe), it is very, very sound advice.

But the rest of the time it's a pretty lousy idea. Working and having fun aren't incompatible--in fact, for most people the two are inextricably linked. Most of us secretly enjoy our jobs--maybe not every single hour of every single day, but enough to take some satisfaction in its daily challenges and its victories, both small and large. Conversely, even when we leave the office, our work habits tend to follow us home; whether consciously or not, we often tackle our pastimes in much the same way we take on our jobs. (There is, after all, a reason that people refer to, say, "working on" a model airplane or a golf swing.)

And so for this edition of our annual Business Life issue, we sought out people who aggressively--some might say obsessively--blur the line between business and pleasure, people whose weekdays are more fun than most people's weekends, and whose idea of taking it easy involves a little hard work. We talked to the folks at MySpace, who, when they're not busy figuring out how to make their site even more addictive, are ... well ... addicted to their own site. We talked to Bill Joy, whose determination and attention to detail built Sun Microsystems, about his quest to build the perfect sailboat--a quest that just so happens to require a great deal of determination and attention to detail. We discovered some rarefied diversions--like collecting and racing Ferraris--that are virtually full-time jobs in themselves. And we asked some business leaders for advice about their hobbies, which yielded a few surprises. (Who would have thought, for instance, that the studio boss responsible for Snakes on a Plane would also have a discerning eye for contemporary art?)

Now, we're not expecting you, as a result of these stories, to go out and build a boat or buy a Ferrari. We're not even expecting you to buy us a Ferrari, although if you feel you must, we won't stop you. (Metallic blue, gray leather interior, please.) Rather, we're asking you to remember that having fun needn't be an impediment to work, and vice versa. Consider this page your permission slip to mix business with pleasure all you want. Except, of course, at next year's Christmas party, in which case we take no responsibility for your actions.