I worked for a small business, we were allowed to have a running bill of products/services. I left my job in January and made a payment on my account. The business owners are now using a friend of mine that still works for the company to relay messages to me to 'warn' me that if I do not pay the remaining balance then they will be 'forced to take action' against me. I have not said that I won't pay the balance but I don't even know exactly how much I owe, and the company has yet to send me anything in writing letting me know how much is owed. I never even got a receipt for my payment that was made. Legally what action can be taken against me? Small claims? collections?
Thanks for any information or help you can give me.

They can take you to small claims court and they may even be able to send your bill to collections. Much of their ability to do the latter depends on whether your debt falls under the specifications of the Fair Debt Collection Act. I found an article regarding collections at: http://www.seniormag.com/legal/debt%20collection.htm