What things should every SQL Server database administrator know?
Books, blogs, tools, you name it.
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.
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.
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):
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.
Don't forget about basic tools
Also check out sqlpass.org as it has a list of most of the SQL DBA Blogs out there.
Don't forget http://twitter.com
Follow SQL guys. SQL Gurus like K. Brian Kelley, mrdenny, Brent Ozar - among others, are all in twitter. Kimberly Tripp & Paul Randal are in twitter too! Follow their tweets. They usually talk a lot about their activities around SQL Server.
Getting to know the DMV's well is on the top of my list. You'd be surprised by how many people overlook those when looking for answers to questions about their servers.
http://www.simple-talk.com is good for the occasional read as well.
This site contains countless tips that have a proven track record:
How to read a query optimization plan and tune queries by re-writing them and/or creating the correct indexes.
Read Kalen Delaney's book on SQL Server Internals.
Itzik Ben-Gan's books and site have also been a big help.
sqlreleaseservices blog for updates.
Many have no clue where they get an overview of what the current cumulative update is - and it shows in the versions they run.