The outstanding ones—in their own words:

Bike Exif
“This site offers a dose of the coolest motorcycles, six days a week. That means new and classic cafe racers, customs, vintage motorbikes, retro track weapons and style icons. I’m passionate about design, photography and classic motorcycles, and Bike EXIF is a way to combine all three—and connect with other people who share the same interests.”

Motorsport Retro
“(…) is an online publication celebrating classic motorsport. It’s about the classic machines, events, design, riders and drivers of the 1960s, 1970s and 1980s. (…) Everything on the site is here because we love it. We are all infected by the motorsport disease. Petrol runs in our blood. We love the sounds, the smell, the speed. We share a passion for those special motorsport moments, the glorious hand built cars and bikes and the gutsy riders and drivers that defined a heady period in motorsport history.”

The Vintagent
—I especially like this note about the site:
“The world of Motorcycles has all the ingredients of a good, enriching drama; heroic deeds, political intrigue, design brilliance, cut-throat business practices, quirky characters, national tensions, cultural biases,eros and thanatos.
When diving into the murk of motorcycle history, we find unexpected riches…everything which makes this life interesting, and worth living.
Motorcycles per se are just metal; it is individuals who animate them, and inhabit the stories within this site. We provide meaning to the metal, and in telling the story of Motorcycling, we tell the story of our world.”