Sheldon Brown
Many cyclists have heard that Sheldon Brown passed away this last weekend.
I only met Sheldon once (at the 2004 Interbike), but he's been a constant and important part of the online cycling community for as long as I've been involved. I was getting serious about cycling in 1996 and that is around the time that he started his famous website and glossary (the glossary started in summer 1996). I didn't like any of the books out there and ended up using some of his articles frequently when learning my way around a bike. I learned a lot from Sheldon's website. He was very generous with his information and never asked for anything in return.
It's an incredible loss.