Free for startups, but larger companies pay: where to draw the line?
pedalpete: For private companies, I suspect you may have an issue dealing with pricing your product based on their revenue.If your product is something that multiple employees use, then it is easier to make the definition based on the number of people.You didn't really give us much to go on. 
Take a look at how companies like 37Signals (basecamphq.com), or freshbooks.com seperate their offerings.Hopefully for you, there is a model which is either # of people using your service, or some other metric you can charge by.For instance, if only 3 people in a small company would use your product, but only 4 people in a large company would use it, then you can't really price based on # of people in the company (or else, how would you know when the company has grown to the new pricing).