How do I add a column, with a default value, to an existing table in SQL Server 2008 or 2012?

ALTER TABLE {TABLENAME} ADD {COLUMNNAME} {TYPE} {NULL|NOT NULL} CONSTRAINT {CONSTRAINT_NAME} DEFAULT {DEFAULT_VALUE}

Add a column with a default value to an existing table in SQL Server

Use ALTER TABLE. For example

ALTER TABLE Table1 
ADD  col2 INT DEFAULT 0;

SQLFiddle