One of my favorite must-have accessories is the Travel Towel. Doesn't seem very exciting, I know, but it is one of the most useful purchases I have made for traveling.
If you travel frequently and are always looking for space saving items, a must have is the microfiber travel towel. Super compact and lightweight, these towels dry extremely quickly and are still able to absorb as much liquid as a regular towel. They can also be used for more than just a towel. I have used mine several times as a blanket and as a makeshift umbrella.
Mine has traveled with me for 10 years now through the Dominican Republic, Ecuador, twice through Europe and other trips, and the thing still looks brand-spanking new. I use a MicroNet Suede Microfiber Towel (you can find them on Amazon here: http://www.amazon.com/MicroNet-Suede-Ultra-Compact-Microfiber-Towel/dp/B003V4MK28