My tomato plants are growing in a green house. It has been raining here for 3 days straight. I check on my plants every morning and today I have this growing on my tomato plant. Help please. I can't find any pictures on the web at all.
Those are adventitious roots. They may occur in damp conditions or when a stem contacts damp earth, and in some cases when the plant has certain problems.
Since you mention three days of solid rain, I assume high humidity in the greenhouse is the reason behind the roots appearing on your plant (or is it more than one plant?). They're just a bit odd-looking; they don't hurt the plant in any way.
Google Images: tomato adventitious roots