Merino wool socks, lifetime warranty. By far the best socks I've owned, and 25% off is matching the best discount I've seen. No coupon code needed for the Darn Tough socks.
SmartWool on sale for 20% off, with code "TREAT". These seem to be more popular than Darn Tough, but IMO Darn Tough socks are better.
Current Promotions on GoBros.com
Darn Tough Sale
: 25% off all Darn Tough socks! No code necessary, discount already applied.
: 10% off sock orders $30+. (code: FEST10). Excludes brands with other current promotions.
Buy 4 Get 1 Free Sale: Buy any 4 pairs of Injinji socks and get the 5th pair on us. Just place 5 pairs of Injinji socks in your cart, type in the coupon code BUY4, and the cost of the lowest priced sock will be subtracted from your cart. This promo works for any increment of 5 Injinji socks (i.e., buy 10 socks and only pay for 8).
Smartwool Sock Sale
: 20% off all Smartwool socks (code: TREAT).