DH and I have thought about this a lot. I wouldn't quit my job right away, but I would call in sick for the rest of the week. I'd hire an attorney and get everything official.
First we would pay off all of our debt (house, car and school loans)
We'd buy land and build a log cabin not too far from where we live now. (Then we'd sell our house.)
I'd replace my car with a new/low mileage Subaru Outback.
DH would probably buy a new truck, but keep his current car.
I'd apply to school for my PhD in psychology or social work.
DH wants to raise buffalo, so we'd build a barn and all that.
If I didn't end up going for my PhD, I would start a therapeutic horse farm for kids with behavioral and developmental disabilities.
We'd pay off the mortgages/school loans for our immediate family members.
We'd set up college funds for my niece and DS.
DH and I would go on the honeymoon that we never go to go on!

That's about it really. We'd also probably have another child sooner than we are currently planning for, since daycare costs and maternity leave wouldn't be an issue. We'd continue to live modestly, but do the things we love to do instead of doing what we have to do to pay the bills.