Professor Steven Jones presenting his X-ray spectrometry evidence (Analysis of molten iron samples taken from WTC's "ground zero.") -See video below.
These samples dramatically show a perfect match for the highly specialized compound "thermate" (used for cutting through steel) found in the WTC debris. (The molten iron samples were gathered from both physical steel and from dust samples blown into the windows of adjacent buildings) If you're new to this information, you might want to check out The 1 Hour Guide to 911 or at least a couple of the essay's sections, like: "Molten Metal" or "Fire Initiated Collapse - Primary Arguments Against"
These "previously molten spheres" consist mainly of iron which, to melt, requires temperatures that far exceed anything jet fuel or burning office materials could produce. (As such, the "official government explanation" ignores irrefutable evidence of incendiaries in the buildings.)
UPDATE: Short video of Richard Gage covering the estimated "10 tons" of iron spheres found in WTC dust samples
UPDATE: May 2, 2009 Professor Niels Harrit (Department of Chemistry at the University of Copenhagen) discusses his recent peer-reviewed paper exposing military-grade nano thermite. (Audio) http://tree3.com/sound/aj/nharrit.mp3
Update: August 2009: A couple clips showing how a building responds when only SOME of its supporting structure is compromised.