Sites
Most of these are multiple purpose. For instance, articles + forums + blogs. This is by no means exhaustive. It's a starting point as you'll find a lot of other blogs and resources from these sites.
Books
This really depends on your role. If you're an operational DBA, you'll want the Wrox book on Administration. If you're dealing with Integration Services, look for the books and resources by SQL Server MVP Brian Knight as a start. If you're doing development, look for SQL Server MVP Louis Davidson's Design book from Apress and SQL Server MVP Grant Fritchey's book on Query Tuning, also from Apress. I'm not very strong on Analysis Services, so I'll leave that for others to recommend. SSRS there are a ton of good books. Also look for the free ebooks at SQL Server Central.
Tools
A lot of good vendors. Start by knowing what comes with SQL Server. Profiler, SSMS, and Configuration Manager you should know cold. BIDS if you do SSIS or SSRS. Otherwise, 3rd parties to look at (in alphabetical order, as people's preferences vary):
Social Networks
There's a group of us fairly active on Twitter. You can find the list of us at SQLServerPedia. Most of us are connected via LinkedIn and FaceBook as well.
Best Answer
Books:
Optimising Oracle Performance - Cary Millsap
Pretty much everything by Thomas Kyte (http://asktom.oracle.com)
Cost-Based Oracle Fundamentals (Jonathan Lewis)
Web sites (if you are dealing with Oracle on Linux):
http://www.puschitz.com - installing oracle on Linux, Linux performance tuning tips
http://www.oaktable.net
...and the oracle websites, metalink etc.