Mozilla labs are promoting a plugin for Firefox that will allow any website to be able to capture audio and video from the browser. But you can already do this with Youtube web-cam upload and many other sites I am sure, but with this add-on you could not be totally sure if the webcam is on or not, and this would not be good for some situations where a minor was using the computer and someone accessed the webcam and was spying on them. But you will just need to be sure if you install this add-on that it is only enabled when you actually want to use it and you disable the plugin when you are done. But there are already websites around where people have hacked into computers and activated the webcam when people are unaware they are being watched.
But on my desktop machine, enabling the plugin would only allow someone to watch my TV tuner and not me anyway, so maybe it would not matter in my case, but for someone with a webcam and a family, do not install this plugin.