I had two friends from MechEngr Graduate School, "Dick & Jane".  They got married.  He worked in manufacturing automation, she worked in software development, both very computer savvy folks.  Jane liked to "garden", that is, plant flowers and shrubs, purchase gnomes, etc.  She started a gardening supplies website in her spare time.  They ran servers from a spare bedroom.  Then they decided to do it full time.  Went big time, hired staff, Jane was Prez and Dick was Chief Tech Officer.  Then they got purchased by BigCorp during the late-90's internet boom.  After the bubble burst 1-1/2 years later the company was closed.  They became very rich in the overall process, but they were forced to dump all of their loyal employees by the BigCorp's corporate chiefs, and Jane took it very hard.

TygerDawg

I own a store on the side.  I work as an engineer from 8 to 4:30.  Before 8 I stop by the store, turn on the computer, crank up the heat, vacuum, etc.  At 4:30 I head to the store and work the evening hours from about 5 til 8 each night.  I sometimes stay later depending on what needs to be done.

I work all day Saturday, every Saturday.  Haven't had one of those off since April 2005.  Most Sundays I go in to clean.   My books are so far behind it isn't funny.  I have 3+ years to pay on my SBA loan.  Financially, it's cost a lot more than I ever expected, in every way.  Someday it will pay off.  I am tough, I'll make it.

My advice would be (1) don't quit your day job until you know it's going to make it.  Start part time so you can at least pay bills.  Cut out every unnecessary expense in your personal life.  Stress over money is a killer.  And (2) go in with a partner.  It's really hard to go it alone.  You need to be able to take a day off here or there.  My family has been really helpful but I still get the feeling like they are thinking, "we told you it wouldn't work..." but it is, I'm just exhausted in making it work.  

My wife and I are about to do exactly this sort of thing.  Packing it up and moving to the middle of nowhere--because we can (beautiful mountain place in CO).  Our dream is for both of us to work in a different business all together--something helping people much more directly.

We don't yet know how to make that pay the bills unless we start running around doing speaking gigs or something--which really isn't what we had in mind.  So for now, I remain an industrial designer while we make the jump to CO and settle in.  We have the start of a three year transition plan where I can begin to ween off of this design business and we'll begin to put more "weight" on the new business plan.

This may be obliquely related--I was given this book yesterday by my "mentee"--looks fascinating, although I've only read the first chapter.  May be something to think of, since the topic deals with such jumps in the market and of marketable thinking generally:
http://www.danpink.com/aboutwnm.php

Jeff Mowry
www.industrialdesignhaus.com
Reason trumps all.  And awe transcends reason.

I used to teach tennis at the local club on the side. I also did the round robin setup, tournament director, that sort of stuff. Keeps me involved I guess.

A couple of other "side" careers that I have heard of:

1) One of my professors worked (owned) a coffee shop. He would invite students over to talk about whatever once a month. Coffee on him.

2) A colleague owned a ranch. I don't know if he ran it or leased it out. I believe it was very profitable.

They did this aside from their main careers - a hobby as it were.

"Do not worry about your problems with mathematics, I assure you mine are far greater."   
Albert Einstein
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