If your goal is to make enough so that you can concentrate on something else, creating a mundane income-producing company is the wrong plan, because it will rapidly stop producing income if you stop paying attention to it.
HotOrNot.com or PlentyOfFish.com have done pretty well for their owners...even if they stopped producing income now (which PlentyOfFish may do, given competition from FaceBook), their founders have banked enough off them to live well for the rest of their lives.
I'd imagine 37signals is in that category too...it sounds like they're making a few million in profit each year.
It depends on your definition of "mundane". You could start a consulting company, take whatever software jobs pay best and make your business grow to the point that it could be attractive to a buyer. This may be boring, but it's easier than creating the next Facebook.
It's also possible to have a mixed model, which is working very well for us. We do some consulting work but only pick clients that are interesting in terms of improving our products or doing work we like. This allows us to be profitable and work on our own interesting stuff without the pressure of concerned investors.
I'd imagine 37signals is in that category too...it sounds like they're making a few million in profit each year.