X-12-ARIMA was the U.S. Census Bureau's software package for seasonal adjustment.
X-12-ARIMA 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.
Notable statistical agencies presently using X-12-ARIMA for seasonal adjustment include Statistics Canada and the U.S. Bureau of Labor Statistics.
X-12-ARIMA was the successor to X-11-ARIMA; the current 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.