For my money, his 49er teams(which really include those under George Seifert too) was the greatest dynasty in NFL history.
I won't post a long tribute here or anything as several people have already done that better and more eloquently than I likely could have. I will however post a link to the amazing Walsh coaching tree. I think it's pretty safe to say that no other coach may ever leave behind so many successful proteges as well as hall of fame players as his legacy. The man truly was a genius and he'll certainly be missed...
The Bill Walsh coaching tree. Which of course our own Andy Reid is a part of.
Niners Nation response to the news.
Story about Walsh from the official 49ers site.
The USA Today took an interesting angle. Imagine what the NFL would be like without Bill Walsh.
Dr Z from SI.com talks about Walsh's take on life in the NFL
Finally the San Francisco Chronicle, The legend that is Bill Walsh