American Apparel makes some of my favorite unisex clothes for babies and toddlers. The quality isn't bad either and there are quite a few organic (sustainable edition) colors available now.
My son wears mostly American Apparel long and short sleeve Ts, sweat pants and zip hoddies. Recently, I purchased the 2T sized thermals (he's only 1 but these fit well). The infant ribbed tanks (aka wife beaters) are cool as well.
They are free of graphics and can be worn big or small, therefore they last an extra long time.