An alarm clock, travel sized and battery powered. Most cell phones have them but in some different countries you need an outlet adapter and current converter to plug the in and charge them. Also make sure you can turn off the roaming function on your phone or leave it on airplane mode so you do not incur roaming fees on your phone plan. I like a small battery powered one because I use my phone for movies and games when I am in the hotel late at night and cannot sleep. It seems like my phone is always dying.
I recommend this item as I have been in a lot of hotels that have power outages (LA in the summer). This way I am more likely to wake up for my flight. I also find that if I am jet lagged I may sleep through one alarm but the second wakes me up.
You can also ask the front desk for a wake up call. Usually at no extra cost just double check first. These too can be unreliable, even when automated.