Timer+

Timer+ turns your iPhone or iPod Touch into a simple countdown timer with unlimited memory. Set timers once for cooking, exercising, laundry, steeping tea, etc., and start them with a tap without having to configure it every time.
Timer+ will alert you even if it is running in the background or the screen is locked, just like an alert for a text message. Be sure to keep your ring/silent switch off to hear the alert sound.