Imceda's auditing solutions are now both available for general download.
SQL Watch will alert via SMTP mail whenever a DDL change occurs on a database. It will alert and also report on the old status of the objects as well as the new status.
SQL Forensics will monitor every command sent to SQL Server and tell you who/what/where/when etc...
So if someone executes any statement (even a select statement or failed transaction) you'll be able to see what user, their IP address, application, etc...
Feel free to send me any questions you have: firstname.lastname@example.org