Excel 2007 : Drop down validation AND dynamic list

Hi,
I would like some help with setting up a drop down list that is managed similar to validation data (restricting the user to only select what is in the list) AND easy to add to the list like (making it dynamic - similar to using the formula, =OFFSET(Sheet1!$A$1,0,0,COUNTA(Sheet1!$A:$A),1)

Any assitance would greatly be appreciated.
Thanks,
Teddy