I'm not a huge fan of top anything lists, but this one is an important one. I'm pragmatic by nature and my must carry list for myself encapsulates that.
1. Swiss Army Knife (or more recently a scary looking letherman.) - I picked up a tiny swiss army knife in Germany during my very first long term backpacking trip in 2004. Somehow I managed to keep track of that same knife through the next 20 or so countries until the present day. I now travel with that one and a larger sized one. The easy answer to why I carry one, is that in any pinch it will help you out. Obviously it's not going to defend you against an attacker or anything like that, but in the event that you get on a train with bread and cheese and no utensils, it can easily fill the void. The scissors will always come in handy at some time or another...don't pull that string on your backpack! There are several tools that can be used to fix things, or pry things out of something, say if something fell in your keyboard. In my opinion if you are creative enough, it is the ultimate MacGyver tool (see note below).
2. Lip Balm, preferably with SPF - This applies to both men and women. Whether on long or short term travel, you never know when you are going to get stuck out in the sun with no shade. Sunburned lips hurt, a lot. I'm a particular fan of Burts Bees.
3. Camera of some sort - This is for obvious travel reasons but I tend to do this in my daily life as well since photography is a hobby. This is much easier now with camera phones, but I used to haul my film camera out with me everyday. What's that old saying, "the best photograph is the one you actually take...
(Extra memory cards are also key for me)
4. Flip flops - Hostel showers, enough said. (Plus my toes like to be FREE!)
5. Journal/Notebook, sometimes both - I'm old school, I still love to journal. I do it any chance I get while on the road and admittedly often fall asleep with said journal in the bed with me or with my face in the journal. It tends to be where I am most honest about everything that is going on around me and inside of me. It is also a great record of a snapshot in time. I also like to carry around a small notebook to take any notes, or write down numbers or directions I might need during the day. A smartphone can also do this nowadays but I don't always travel with a smartphone.
6. Duct tape - Now that I have written that, I am not sure it is a universal term, I've never looked for duct tape in another country because I always have it with me. Again, this is for MacGyver type reasons. It can be used for any situation you get stuck in. It might sound silly, but you will no doubt use it, especially if you pull that loose string on your backpack :)
7. Lock - It's heavy so some people opt not to carry it, but it is worth it when you need it, especially in hostels.
Secondary items - B List items
1. Paperclips
2. Matches or Lighter
3. Mini packets of tissues
4. Safety Pins
5. Bandana
6. Plastic Bags - Ziploc or the regular kind that stores give out or both.
7. Microfiber Towel
8. Dr. Bromers Liquid Soap
9. Flashlight
10. Mini First Aid Kit
New Addition
1. Doorstop - At the suggestion of other female RTW-ers I have started carrying a doorstop. They say it helps prevent someone from getting in a room. I'm not sure if it works but I figured I would try it out. I used to just pile my pack against doors, but this seems a bit more space economical.
In case you can't tell from this post, I am the daughter of an engineer and MacGyver-ing things is sort of a specialty of my upbringing. (For those who don't know, MacGyver was an action TV character in the 80s & 90s in the U.S. He was very resourceful and could rig anything to help him in a tough situation. I have never seen a full episode of the show, but it's lore is regular part of the pop culture in the U.S.).