Excel has a limited way of doing this.
In xl2003 menus:
Tools|Options|Edit tab|check "AutoComplete for cell values"
It uses the recent cells above the cell you're entering data as the list to
choose from.
If you only need this for a few entries, you may find that it's irritating to
toggle on/off.
You can rightclick on a cell and choose "Pick from drop-down list". It may be
sufficient for you.