WebJack enables communication between an #arduino and a web browser, without any plugins. A cheap Arduino could be plugged into another Arduino to listen to it's serial communications, relaying them back and forth with a browser. This would be helpful for quickly connecting with any serial-enabled device (like an Arduino) without a serial driver on your computer. You could even do it via a phone -- any device that has a headphones port and a web browser.
This example using webjack-firmata implements serial communication via firmata's serialWrite, but depends on StandardFirmataPlus.ino, which has not yet been ported over to be WebJack compatible. This could allow webjack-firmata to act as a serial bridge.