I've been wearing 100% boxer briefs for several years now, and up until recently, I never really paid much attention to the swamp butt they would give me in the southern heat.  I run about 20 mi/wk and wear synthetic clothing to wick sweat, so why not synthetic underwear?  I bought medium this time around, as I'm down to a 32 waist; however, I'm going to pick up a pair of smalls next time around.  The mediums didn't feel loose, but they didn't feel as snug as I hoped for.

I found them at my local jcpennys yesterday for $20/pack of 2. or if you buy two packs (4 pair), they are $15/pack.  I paid $24 for 1 pack (2 pair) here, on amazon.