My mother passed in 2006, there was no estate. She hadn't filed taxes in over 10 years because she did not qualify to do so. Just received 1099-C form for her estate. What do I do with it?

1099-misc is your mother's income in 2008. She may have a tax due. But since she does not have any estate, you are not responsible for paying the tax.
Let us see what other tax experts have to say.

I suspect what happened is her estate earned some income (after she passed away) and that income is being passed on to the heirs (you). Ask the executor of the estate about this - he should be able to explain why this income is being passed through by the estate to you.