For Triathlons: Nike Vision Tailwind 12

These frames are light (24 grams), comfortable, and both wind and fog-proof – making them just right for a triathlon. They keep the air out of your eyes on the bike ride, but the ventilation slits that run along the top of the lens make sure they won't fog up on the run after. If your tri or road race has ample sunny asphalt, we also recommend getting the Speed Tint lenses, which minimize the asphalt-specific glare. 

[Nike.com, $99 base, $120 with Speed Tint lenses]