Shopnote

This extension displays a configurable hint in the head area of the shop. You can determine a startdate and an enddate. There's also the possibility to show the message in a popup. The popup shows up only once per note until you change the title or the customer clears his cookies.

For example you can inform your customers about special offers, your holidays, a planned maintenance or whatever you want.

Demo:
Frontend
Backend

Further information:
http://www.magemallow.com/shopnote.html