These briefs are designed for travel. They are quick drying, breathable, lightweight, moisture wicking, and anti-microbial.
There are women's and men's styles. They are not inexpensive, but they seem to last a long time and they are extremely convenient because you only need to take two pairs no matter the length of your trip. Each night, you can wash a pair then squeeze them in a towel and hang them in a well ventilated area. My experience is that they are always dry by the following morning (I have not used them in any extremely cold or humid climate yet, but I expect them to behave better than traditional underwear in any climate).
The Give-N-Go Briefs are 94% nylon and 6% spandex. They are very comfortable. I was actually told about these by a male friend, and the drying properties are probably more important to men since a lot of women's underwear dries quickly anyway.
The women's briefs come in a various styles such as bikini, full-cut, string bikini, and thong. They are available in a variety of popular colors. I bought mine at REI, but they are also available at many quality sports stores, the Ex-Officio and other websites.
You can pack lighter with these quality travel briefs.
Update On Feb 09, 2010: I should have mentioned that I use Classic Ivory Bar Soap (with good results) for these and most of the sink washing that I do when travelling.