Mike Cohn - A great mentor, trainer and all around SCRUM guru
Scott Ambler - Words of wisdom on Agile processes and modeling
Craig Larman - Worked with Craig at ObjectSpace - read his books!
Alistair Cockburn - That's pronounced "Coburn", the Scottish way
UML - The Object Management Group
IBM Rational - The new home of the RUP
Agile Alliance - Good source of information on all things Agile
SCRUM Alliance - Obviously a source of SCRUM stuff
APLN - Agile Project Leadership Network - find you local group
APLN Dallas - My local group
APLN Fort Worth - The one across the Trinity - new, so no link yet