Auto form Fill

I want to write a program in Javascript that automatically fill in the user name and password in text fields of a live web page. Then it logs in for me.

The only way I can change text fields right now is to save the source page and modify code and then run the new code. But I want to be able to initiate a script that will fill out appropriate user name while I am at that website.

Can you guys tell me the basic implementation. I will figure out the details.

All Javascript scripts are constrained by the same origin policy: scripts from one Web site do not have access to information such as usernames, passwords, or cookies sent to another site.