Got this comprehensive list from Michael Woods at Microsoft:
BizTalk Server website on Microsoft.com:
http://www.microsoft.com/biztalk/default.mspx
Business Process and Integration developer center on MSDN:
http://msdn.microsoft.com/biztalk/default.aspx
BizTalk Server hands-on, online Virtual Lab:
http://msdn.demoservers.com/login.aspx?group=biztalk
BizTalk Server 2004 Service Pack 1
http://www.microsoft.com/downloads/details.aspx?displaylang=en&FamilyID=f4a5ab9e-d599-4cc8-abdf-ae6ae68bac3d
Latest updated documentation online:
http://msdn.microsoft.com/biztalk/default.aspx?pull=/library/en-us/def/htm/ebiz_def_portal_page.asp
Latest updated documentation download:
http://www.microsoft.com/downloads/details.aspx?FamilyId=CDD47461-F4E2-4BC6-B5C2-2018AFF2823D
Software Developer Kit Refresh:
http://www.microsoft.com/downloads/details.aspx?FamilyId=8A1CA3AF-790C-4261-838A-9F0661C72887
Updated Tutorials (The labs from the QuickStart are based on these tutorials):
http://www.microsoft.com/downloads/details.aspx?FamilyId=8A1CA3AF-790C-4261-838A-9F0661C72887
Integration Patterns
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnpag/html/intpatt.asp
Deployment Guide for Secure Deployment
http://msdn.microsoft.com/library/en-us/BTS_2004WP/html/ace6e1b9-ca24-4d14-9203-099276b50a73.asp?frame=true
Technical Guide for High Availability
http://msdn.microsoft.com/library/en-us/BTS_2004WP/html/922d9b1a-c6f2-4d42-9f7a-2876eac7f50d.asp?frame=true
The community:
http://www.microsoft.com/biztalk/Community/default.mspx
BizTalk Newsgroups
http://www.microsoft.com/technet/community/newsgroups/server/biztalk.mspx
Scott Woodgate’s Weblog:
http://blogs.msdn.com/scottwoo/
Jon Flander’s Weblog:
http://www.masteringbiztalk.com/blogs/jon/default.aspx
Got Dot NET community site:
http://www.gotdotnet.com/team/wsservers/
The Bloggers Guide to BizTalk
http://www.gotdotnet.com/Workspaces/Workspace.aspx?id=0dfb4f4e-d241-4bc8-8418-2c385d8e3eaa
BizTalk 2004 Performance Tuning Whitepaper & Utility
http://www.gotdotnet.com/team/wsservers/bts2004/BTS2004Performance.zip
http://www.gotdotnet.com/community/workspaces/workspace.aspx?id=ca5285b6-8657-4468-9462-e36b06b3dbeb
BizTalk Enterprise Patterns:
http://www.gotdotnet.com/workspaces/workspace.aspx?id=3ac7ecdf-4478-4cfc-8966-029709115eac