Sqlmap is awesome, that's all you need to know.

It will basically check a website and try or allow you to inject sql query's into the sites backend database.
If successful you could use it to dump all information in said name database that you are looking at.
This can include but is not limited to: usernames, passwords, email addresses, customer information, etc.. etc...
Sqlmap is also able to be used or integrated with a variety of other applications and attacks such as using it in combination with metasploit and possibly even nikto or nmap would yield great results. =]

FYI... this really great when they happen to be running an ldap server and the usernames are not just for logging into a web application or the database to alter files but are actually for the system itself!