I need a tool for keeping track of the usage of the internet?

I looking for a tool where I can download it and keep track of my internet usage.
I am a big user of the internet and I want to see how how much I am using, so I know how much my bill payments are going to be. I am on Broadband ( Smartchat) unlimited downloads.
I have looking on the web for that type of tool but I can’t seem to find it.
I don’t want to spend big bucks for it, is there such a tool any
free downloads I can download, I think it like a countdown tool it tells you how much time you have got before it shutdown?