Solicitação HTTP simples como promessa

function http(options) {
var deferred = q.defer(),
req
= new XMLHttpRequest();

if (!options.url) {
throw new Error('No url given');
} else {
req
.open(options.method || 'GET', options.url, true);
}

// Set request headers if provided
Object.keys(options.headers || {}).forEach(function (key) {
req
.setRequestHeader(key, options.headers[key]);
});

req
.onreadystatechange = function (e) {
if (req.readyState !== 4) { return; }

if ([200, 304].indexOf(req.status) === -1) {
deferred
.reject(new Error('Server responded with a status of ' + req.status));
} else {
deferred
.resolve(e.target.response);
}
};

req
.send(options.data || void 0);

return deferred.promise;

}

Com suporte amigável de Matthew Phillips:
https://gist.github.com/matthewp/3099268