Cuppa is a free program to tell you when your cup of tea is done steeping.
Use it from the Dock: just right-click or control-click on Cuppa’s icon and select the beverage you are brewing. Or select a beverage from the Beverages menu. Cuppa will begin timing the brew, and you’ll see a teabag appear in the cup and gradually darken as the tea steeps. When the tea is done Cuppa will attempt to get your attention by various configurable means. You can also enable a countdown timer that’s displayed in the dock icon.