My PR firm was originally offline when I focused only on music publicity (now it's almost entirely online though). I barely spent a cent to start the business, even offline, other than some basic office supplies. That's the great thing about my industry though. When the business world is going good, people need you to announce their good news. When the economy or other things go sour, they need us even more to fix their problems.

Any kind of consulting should be a low-start up cost kind of business... unless of course it's in a field where you need to worry about professional licenses and such (legal, medical, architects,etc.)

Here's a list on one of my sites with 65 different possible consulting jobs you could consider going into.

Jenn