...DSTK Engine and DSTK ScriptWriter are opensource, but DSTK Studio and Text Explorer requires small amount of payment. DSTK Studio and Text Explorer are free to use 10 times
jBilling is a web-based enterprise billing system.
jBilling is a web-based enterprise billing and rating system. It manages your subscribers with automatic invoicing (email and PDF) and paymentprocessing (credit cards, checks, direct deposit). Robust, well documented and easy to use!