Well, to take the question literally, my asthma inhaler. I'd probably die without it because my asthma can get pretty bad, sometimes even to the point that the inhaler is only enough to keep me breathing very shallowly long enough to get medical assistance.
Aside from that, I'll second Brovo's pick of alarm clocks and other alarm-capable devices. I just cannot manage to maintain a normal sleeping schedule without an alarm clock, and the only totally effective method I have for remembering to do minor things that need doing is to set an alarm for it to remind me. I'm good about things like doctor appointments, but remembering to take out the trash and recycling on the right night of the week? It would never happen without my phone alarm.