哪里可以找到带有灰色指令的漂亮文本框？

While logging in to msn-messenger you have these nifty textboxes that show you what to type (username, password) in gray text. Once you start typing it disappears. This surely is not hard to implement using a user control in WPF, but is this control/behaviour already available? While logging in to msn-messenger you have thes