BizTalk Server has a vibrant community full of smart folks offering up tricks, tips and gems. Some great sites include the following listed in no particular order and definitely incomplete: 

•  BizTalk Server at Codeplex.
• The BizTalkDisassembler add-in for .NET Reflector is a great addition to one of my favorite tools on the net. Use the BizTalkDisassembler to sniff around BizTalk application assemblies to learn how a solution is compiled or to better understand an existing solution.
• BizUnit provides a solid framework for unit testing your BizTalk Server solution. Download the framework and leverage it in your next BizTalk Server project or use it to verify enhancements to an existing solution.
• Automate BizTalk Server tasks using the BizTalk Server provider for PowerShell. You use PowerShell, right?
• Got orchestrations?  Profile them using the BizTalk Server 2006 Orchestration Profiler. As the project page says, "this tool can be run to gain a consolidated view of orchestration tracking data for a specified period of time to help developers get an idea of how their orchestrations are being processed and how much coverage they are getting in their testing".
• The BizTalk Server Pattern Wizard provides a way to capture and reuse orchestration patterns. This is a great tool for ensuring orchestration uniformity across projects and can be easily extended as needed.
• Have the BizTalk Server 2006 Documenter capture your application details in a chm for posterity (and baselining). This is one of those "well duh" tools and is a natural companion to BizTalk Server...like one of those buddy-buddy flicks. Maybe Eddie Murphy in Beverly Hills Cop. Maybe.
• Check out the TCPIP Adapter, the ADO Adapter, the Patterns and Practices Enterprise Service Bus guidance, and other projects.
• BizTalk Server Newsgroups. Participate in the BizTalk Server newsgroups to find answers to common questions, discover tips and tricks, and contribute expertise to the community. 
• Blogs
• BizTalk Server Team Blog, a great place to find insights about BizTalk Server and the business process management space straight from the product group! Recent posts include information about TechEd US 2007 and BizTalk Services. Take a look!
• Random ramblings about Technology, a blog with material on BizTalk Server, WCF and other technology by Martijn Hoogendoorn.
• Jesus Rodriguez WebLog. Recent posts include bits on Windows Workflow and the SQL Server Service Broker, the Orca's Json Serializer, and the WCF NetMSMQ adapter.
• Jeff Lynch. Jeff has a great post on mapping the OrderGroupCollection schema in Commerce Server 2007.
• Matt Meleski's .Net Blog - The ABC's of .NET. Recent posts include BizTalk message helper methods and turning an immutable message in BizTalk into a mutable message.
• Jon Flanders' Blog:BizTalk and Windows Workflow Foundation. Jon has a great post on orchestration performance.
• Eric Stott's BizTalk and HL7 Blog. Eric has been busy providing great information about the cross-referencing functoid and associated API/tools and HL7. If you work with EDI, visit Eric's blog.

If I forgot your blog or your favorite site, drop me a line and I will put them on "part 2".