First of all, here is the packed source for your bookmark:
javascript:(function() {var URL = 'https://store.steampowered.com/account/registerkey';if (document.location.href != URL) {document.location = URL;alert("Redirecting to the proper page. Run again");return;}async function registerKeys(keys) {while (keys.length) {var key = keys.shift();try {var success = await registerKey(key);console.log("registered " + success.purchase_receipt_info.line_items[0].line_item_description);} catch(e) {failures.push(key);OnRegisterProductKeyFailure( e.purchase_result_details, e.purchase_receipt_info );switch(e.purchase_result_details) {case 9:console.error("You already own " + e.purchase_receipt_info.line_items[0].line_item_description + " - Save the key " + key);break;case 53:console.error("You've hit the API too much for now. Try again later");default:keys.unshift(key);console.log("Still unregistered: ", keys);return;}}}}function registerKey(key) {return new Promise(function(resolve, reject) {new Ajax.Request('https://store.steampowered.com/account/ajaxregisterkey/',{method:'post',parameters: {'product_key' : key,'sessionid' : g_sessionID},onSuccess: function(transport){if ( transport.responseText ){try {var result = transport.responseText.evalJSON(true);} catch ( e ) {return reject( 0, null );}if ( result.success == 1 ) {return resolve( result );} else if ( result.purchase_result_details !== undefined && result.purchase_receipt_info ) {return reject( result );}}return reject( 0, null );},onFailure: function(){return reject( 0, null );}});});}var toreg = prompt("Enter your keys to register, seperated by a space").trim().split(/[^-\w]+/).filter(v => !!v);if (toreg.length) {registerKeys(toreg);} else {alert("No keys entered");}})();
Steam recently added a page to register keys on the website. Under the hood, its just making requests to https://store.steampowered.com/account/ajaxregisterkey/ to register a key. Now, I'm sure this could be more automated, or a script or program, but I threw this together to use myself after purchasing a good 30+ games, and figured I would share with the world.
This should work in most modern browsers, see http://caniuse.com/#feat=async-functions Errors are a little obtuse, you'll want to watch the developer console to watch the status updates.