Wool Poncho aka The Drinking Rug
I've taken the Drinking Rug on every journey that I've taken.  I've found it incredibly useful, as a mat or blanket, as well as a very warm poncho.  It's also completely waterproof, though it does take about a day to dry, longer if you're in a rainforest. 
Photo:  Greece 2001
Photo:  Suva 2004
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