I've been making my way through the batch of newly released videos by NIST and I thought these were quite interesting. They show multiple 'ejections' (for lack of a better word) from the WTC North Tower with a white trail of smoke burning behind them. Contrast these ejections with multiple other objects and debris falling from the tower that do not have any smoke trails, suggesting that there is something different about the 'white trail' objects. One hypothesis: We know that smoke from thermitic reactions burns white, and we know (e.g., Harrit, Jones, et al.) that thermite is prevalent in the dust from the collapses. Perhaps that's what you're seeing in this video.

In the second clip ("NY1 6 Raw 10"), pay attention to the first ejection, which falls at :32 into the video. It seems that there is no white smoke at first, but that as it's falling, the smoke starts. Perhaps a delayed thermitic reaction? Then you see more of the same on the right side of the video.

I found these clips in the 9/11 archive that was published by archive.org. These videos were taken from the high quality MPEGs that were available for download for some time. Links are provided to high quality copies of this footage as well as links to the streaming versions on archive.org

Showing the South side of WTC7, broadcasted by NBC at 1:45 PM.

http://video.google.ca/videoplay?docid=6186921835292416413

high quality XVID (640x480 de-interlaced) :
http://www.megaupload.com/?d=CV4PU0IQ

link to archive.org (@ 22 minutes):
http://www.archive.org/details/abc200109111323-1404

What is that black marking? It looks like a hole, but it seems much too smooth for being caused by falling debris. It looks to be at least 15 stories high.

Screencaps: