From Jimbojw.com

// This is a Greasemonkey Userscript. You may choose to install it or download the raw source.

Important! Unless otherwise specified, this userscript is released under The MIT License. If you choose to install it, you do so at your own risk and discretion.

// GMHttpRequest
// version 0.1
// Copyright (c) 2007, Jim R. Wilson (wilson.jim.r at gmail)
// Released under The MIT License (http://www.opensource.org/licenses/mit-license.php)
//
// Purpose:
//    This userscript injects a class called GMHttpRequest which behaves
//    like the native XMLHttpRequest, except with none of the cross-domain
//    limitations.
//
// WARNING:
//    This script must be kept limited to TRUSTED PAGES ONLY.  Allowing
//    GMHttpRequest to execute within arbitrary pages is DANGEROUS since
//    it opens the user to all kinds of cross-domain vulnerabilities.
//    GMHttpRequest is designed as a proof-of-concept only, and should
//    not be relied upon directly for anything other than development
//    work and research.
//
// --------------------------------------------------------------------
// Copyright (c) 2007 Jim R. Wilson
// 
// Permission is hereby granted, free of charge, to any person 
// obtaining a copy of this software and associated documentation 
// files (the "Software"), to deal in the Software without 
// restriction, including without limitation the rights to use, 
// copy, modify, merge, publish, distribute, sublicense, and/or 
// sell copies of the Software, and to permit persons to whom the 
// Software is furnished to do so, subject to the following conditions:
// 
// The above copyright notice and this permission notice shall be 
// included in all copies or substantial portions of the Software.
// 
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, 
// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES 
// OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND 
// NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT 
// HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, 
// WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING 
// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR 
// OTHER DEALINGS IN THE SOFTWARE.
// --------------------------------------------------------------------
//
// ==UserScript==
// @name           GMHttpRequest
// @namespace      com.jimbojw
// @include        http://jimbojw.com/examples/gmhttprequest_demo.html
// ==/UserScript==
 
// Inject a reference to the GM_xmlhttpRequest function directly into the TRUSTED PAGE
// Note: This is what makes the script so dangerous - please see the WARNING above for more info.
unsafeWindow.GM_xmlhttpRequest = GM_xmlhttpRequest;
 
// Inject the GMHttpRequest Class
var script = document.createElement('script');
script.appendChild(document.createTextNode(new String( (function() { (function() {
 
// Class code borrowed from Mootools (with modifications)
// See: http://docs.mootools.net/Class/Class.js
var Class = function(properties){
    var klass = function(){
        if (this.initialize) return this.initialize.apply( this, arguments[0] !== null ? arguments : null );
        return this.abort ? this.abort.apply(this) : this;
    };
    klass.prototype = properties;
    klass.constructor = arguments.callee;
    return klass;
};
 
var GMXHRWrapper = new Class({
    headers: { },
    initialize: function( instance, method, url ) {
        this.instance = instance;
        this.method = method;
        this.url = url;
    },
    request: function( options ) {
        if (!options) options = { data: null };
        var instance = this.instance;
        options.url = this.url;
        options.method = this.method.toUpperCase();
        options.headers = this.headers;
        options.onreadystatechange = function(responseDetails) {
            var ghr = window.GMHttpRequest.instances[instance];
            if (!ghr) return;
            for (var i in responseDetails) ghr[i] = responseDetails[i];
            if (ghr.readyState==4) {
                ghr.responseXML = null;
                try {
                    var ctype = ghr.getResponseHeader('Content-type').match( /^(.*?)(;|$)/ )[1];
                    if ( ctype=='text/xml' || ctype=='application/xml' || ctype.match(/\+xml$/) ) {
                        var dp = new DOMParser();
                        ghr.responseXML = dp.parseFromString( ghr.responseText, ctype );
                    }
                } catch (err) { }
            }
            if (ghr.onreadystatechange) ghr.onreadystatechange();
        };
        if (options.method.match( /^(GET|HEAD)$/ ) && options.data) {
            var ch = options.url.indexOf('?')==-1 ? '?' : '&';
            options.url += ch + options.data;
        }
        GM_xmlhttpRequest( options );
    },
    setRequestHeader: function( header, value ) {
        header = header.toLowerCase();
        this.headers[header] = value;
    }
});
 
window.GMHttpRequest = new Class({
    abort: function(){
        if (typeof this._ghrwrapper != 'undefined') delete this._ghrwrapper;
        if (typeof this.instance != 'undefined') {
            window.GMHttpRequest.instances[this.instance] = null;
            delete this.instance;
        }
        this.readyState = this.status = 0;
        this.responseText = this.statusText = this.responseHeaders = '';
        this.responseXML = null;
    },
    getAllResponseHeaders: function(){ return this.responseHeaders; },
    getResponseHeader: function( header ){
        header = header.toLowerCase();
        var pairs = this.responseHeaders.split("\n");
        for (var i=0; i<pairs.length; i++) {
            if (pairs[i].substr(0,header.length).toLowerCase() == header) {
                var m = pairs[i].match( /.*: (.*)/ );
                return m ? m[1] : null;
            }
        }
        return null;
    },
    onreadystatechange: null,
    open: function(method, url, async, user, password){
        this.abort();
        this.readyState = 1;
        this.instance = window.GMHttpRequest.instances.length;
        window.GMHttpRequest.instances.push( this );
        if (this._ghrwrapper) delete this._ghrwrapper;
        this._ghrwrapper = new GMXHRWrapper( this.instance, method, url );
        if (method.toLowerCase()=='post') this.setRequestHeader( 'Content-type', 'application/x-www-form-urlencoded' );
    },
    send: function( data ){
        this._ghrwrapper.request( { data : data } );
    },
    setRequestHeader: function(header, value){
        if (this.readyState != 1 || !header || !value) return;
        this._ghrwrapper.setRequestHeader( header, value );
    }
});
window.GMHttpRequest.instances = [];
window.GMHttpRequest.version = '0.1';
 
})() }) ).replace( /^function \(\) \{\s([\s\S]*)\s\}/m, '$1' )));
document.getElementsByTagName('head')[0].appendChild(script);
 
//