Firefox: Control+Page Up, Control+Page Down

Safari: ⌘{ and ⌘}

Camino: ⌘⌥← and ⌘⌥→

Adium: ⌘← and ⌘→ by default, ⌘[ and ⌘]

iChat: ⌘{ and ⌘}

TextMate: ⌘⌥← and ⌘⌥→

Dreamweaver: ⌘` (which is wrong, that is for switching windows)

I don’t know what the appropriate response is: Argh, groan, or sigh.