X-12-ARIMA is the U.S. Census Bureau's software package for seasonal adjustment. It can be used together with many statistical packages, such as Gretl or EViews which provides a graphical user interface for X-12-ARIMA, and NumXL which avails X-12-ARIMA functionality in Microsoft Excel.
X-12-ARIMA is the successor to X-11-ARIMA. The next version is X-13ARIMA-SEATS.
Both X-12-ARIMA and X-13-ARIMA-SEATS's source code can be found on the Census Bureau's website.