Well, I always receive this question again and again, specially when delivering talks on technical communities. Veteran DBAs may just simply doing bing or googling, most of newbie find themsselves overwhelmed by tons of info from the net.
Let me share my favourite site that I always visit regularly:
I subscribe to their newsletter since 2001, and never get bored since then. I even wrote 2 aqrticles for them, but now I just do ot have enough time anymore. This is the biggest SQL articles repository in the net, with very active community. SQL MVPs Brian Knight, Andy Warren, and Steve Jones are the founder and managing the site.
He is a SQL MVP and MCM as well, posted many interesting stuff on IO planning, virtualization and other random stuff
It containts the most advanced SQL Server stuff on the planet, maintained by highly respected SQL expert Paul Randall and Kimberly Trip. Paul is the creator of DBCC CheckDB when he was with Microsoft.
Technet and MSDN
Many resources such as video, how to, and whitepaper.