Street worthy thermal clothing manufactured in New Zealand from the wool of Merino sheep.
The difference between Icebreaker and normal wool thermal kit is that it does not itch, can be worn next to the skin and it doesn't smell ever, not even after a month, unlike polypropylene which takes on the wearers body odour after about a week and stays that way forever.
Maybe it is a bit disgusting to contemplate, but a friend wears hers to cycle to work in, then wears it at work (and she is a Fund Manager so needs to look snappy) and only washes it every week or two and then out of a sense of duty more than a need to do it.
The wool from the Merino sheep has very fine fibres which are about 1/4 the diameter of normal wool. This means that very light but warm garments can be made. The fibres can absorb about 1/3 their own weight in water which can then be released to the atmosphere without the wearer feeling wet and clammy.