Internet usage monitoring programs?

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Soon I shall be getting onto a new internet plan. With my current plan, if I go over the usage amount, I was have my speed sent to dial up speed. With this new plan, however, if I go over, I will automatically get another ten GB's, but charged for it. I don't want this, so I have worked out how much I can use a day so this won't happen. The problem is enforcing this. I need a program that can warn me when I am near using up the day's current amount, perhaps even disconnect me. Does this sort of program even exist? There is the company's usage meter, but that isn't live.