Looks in your "To" field and sees if you have an email address in your "From" addresses with the same domain. Changes <select> onblur

gmailfrom.user.js

// ==UserScript==
// @name         Gmail "From" Address auto-selector 
// @version      0.17
// @description  Looks in your "To" field and sees if you have an email address in your "From" addresses with the same domain. Changes <select> onblur
// @author       Andrew Sutherland, https://github.com/asuth
// @include      http://mail.google.com/*
// @include      https://mail.google.com/*
// @include      http://*.mail.google.com/*
// @include      https://*.mail.google.com/*
// ==/UserScript==

(function() {
	
	var blurHandler = function() {
		
		var domains = {'' : 0},
			mostCommon = '';
		
		// parse email addresses in TO: field, find most common domain
		this.value.split(',').forEach(function(str) {
		
			var addr_part = str.split('@');
			if (addr_part.length > 1) {
		
				var domain = addr_part[1].split('>')[0];
				domains[domain] = domains.hasOwnProperty(domain) ? domains[domain]+1 : 1;
		
				if (domains[domain] > domains[mostCommon])
					mostCommon = domain;
		
			}
		
		});
		
		if (mostCommon == '')
			return;
		
		var form = this,
			c = 0;

		while (form.nodeName != 'FORM' && c++ < 20)
			form = form.parentNode;
	
		if (!form.hasOwnProperty('from'))
			return;
		
		// select the first option with a matching domain name
		var from_options = form.from.options;
		for (var i = 0, m = from_options.length; i < m; i++) {
			if (from_options[i].text.indexOf(mostCommon) > -1) {
		
				from_options[i].selected = true;
				break;
		
			}
		}
		
	};
	
	// wait for the compose field to show up
	// never clear this interval because we might compose multiple 
	// emails and we have to target new copies of the form
	var interval = setInterval(function() {
	
		var forms = document.getElementsByTagName('form');
		for (var i = 0, m = forms.length; i < m; i++) {
						
			if (!forms[i].hasOwnProperty('to'))
				continue;

			// addEventListener discards exact duplicates, so don't worry about 
			// adding many times
			forms[i].to.addEventListener('blur', blurHandler, false);			
							
		}
	
	}, 1500);

})();