The Code

This user script runs on all Yahoo! pages, but the first thing it does is check for the existence of the Yahoo! login form. If it doesn't find a login form, it just exits without doing anything. If it does find the login form, it sets a short timer to replace the username input box with a drop-down menu of your Yahoo! IDs.

Edit the following user script to include your Yahoo! IDs, and then save it as yahoo-select.user.js:

	// ==UserScript==
	// @name         Yahoo! User Persitance Thing
	// @namespace    http://www.rhyley.org/gm/
	// @description  Add a drop-down box to the Yahoo login form
	// @include      http*://*.yahoo.tld/*
	// ==/UserScript==

	// based on code by Jason Rhyley
	// and included here with his gracious permission

	// ** Replace this array with your Yahoo IDs **
	var gUserIDs = new Array("Put","Your","User","ID","Here");

	var login = null;
	var password = null;
	
	function buildLoginThing( ) {
		if (gUserIDs[0] == 'Put'){
			alert('You must configure the script before it will \n' +
				  'work propery. Go to "Manage User Scripts" and\n' +
				  'click the \"Edit\" button to configure the script.');
			return;
		}

		var elmSelect = document.createElement("select");
		elmSelect.id = "username";
		elmSelect.name = "login";
		elmSelect.className = "yreg_ipt";
		elmSelect.addEventListener('change', function( ) {
			if (this.selectedIndex == this.options.length-1) {
				window.setTimeout(function( ) {
				var elmNew = document.createElement("input");
				elmNew.type = "text";
				elmNew.id = "username";
				elmNew.name = "login";
				elmNew.className = "yreg_ipt";
				login.parentNode.replaceChild(elmNew, login);
				login = elmNew;
				login.focus( );
				}, 0);
			} else {
				password.focus( );
			}
		}, true);
		var arOptions = new Array( );
		for (var i in gUserIDs) {
			arOptions[i] = document.createElement("option");
			arOptions[i].value = gUserIDs[i];
			arOptions[i].text = gUserIDs[i];
			elmSelect.appendChild(arOptions[i]);
		}
		arOptions[i] = document.createElement("option");
		arOptions[i].text = "Other…";
		elmSelect.appendChild(arOptions[i]);
		login.parentNode.replaceChild(elmSelect, login);
		login = elmSelect;
	}

	if (document.forms.length) {
		for (var k = 0; k < document.forms.length; k++) {
			var elmForm = document.forms[k];
			if (elmForm.action.indexOf('login.yahoo.com') != -1) {
				elmForm.addEventListener('submit', function(e) {
				e.stopPropagation( );
				e.preventDefault( );
				}, true);
				login = elmForm.elements.namedItem('login');
				password = elmForm.elements.namedItem('passwd');
				break;
			}
		}
	}

	if (!login) { return; }
	if (location.href.indexOf("mail.yahoo.com") != -1) {
		location.href = "http://login.yahoo.com/config/login?.done=" +
			"http%3a%2f%2fmail%2eyahoo%2ecom";
	} else {
		buildLoginThing( );
		setTimeout(function( ) { password.focus( ); }, 100);
	}