Microsoft Coding Corner: http://www.microsoft.com/isaserver/techinfo/Guidance/2004/coding.mspx
Jim Harrison's ISA Server Tools Repository: http://isatools.org/
Jason Fossen's ISAscripts.org: http://www.isascripts.org/
ISA Server 2004 Software Development Kit: http://www.microsoft.com/downloads/details.aspx?FamilyID=&displaylang=en
Jespen Hanno Hansen's website at http://www.hanno.dk