Chrome/Firefox/Internet Explorer: Facebook Chat now includes a feature that lets you know when a friend has read your message -- or when you've read theirs. If you'd prefer to keep that information under wraps, Chat Undetected can do it for you.
Nothing is more annoying than when someone messages you on Facebook when you don't want to talk to them. Unfortunately, now they'll know when you're really there, because Facebook tells them so. To fix this, just install Chat Undetected in your browser and they'll never be notified of which messages you have and haven't seen. If you don't want to talk to anybody, you can always just turn Facebook Chat off too.
Chat Undetected is a free extension for Firefox, Chrome and Internet Explorer.