Microsoft ISA Server 2004 product overview: http://www.microsoft.com/isaserver/evaluation/overview/default.mspx
What's new in ISA Server 2004: http://www.microsoft.com/isaserver/evaluation/whatsnew.mspx
Comparison of ISA Server 2004 Standard and Enterprise Editions: http://www.microsoft.com/isaserver/evaluation/se_ee_comp/default.mspx
ISA Server 2004 overview presentation: http://www.microsoft.com/isaserver/evaluation/demo/default.mspx
ISAserver.org: http://www.isaserver.org/
German ISA Server FAQ: http://www.msisafaq.de/
ISA Server Community: http://www.microsoft.com/isaserver/community/default.mspx
ISA Server 2004 technical resources: http://www.microsoft.com/isaserver/techinfo/default.mspx
ISA Server books: http://www.microsoft.com/isaserver/techinfo/books/default.mspx