As it was mentioned above CasperJS is the best tool to fill and send forms. Simplest possible example of how to fill & submit form using fill() function:
casper.start("http://example.com/login", function() {
//searches and fills the form with id="loginForm"
this.fill('form#loginForm', {
'login': 'admin',
'password': '12345678'
}, true);
this.evaluate(function(){
//trigger click event on submit button
document.querySelector('input[type="submit"]').click();
});
});