I just glanced at the Azureus site on SourceForge. In the past 10 days, 3.2 million people have downloaded the .torrent file for the Azureus 188.8.131.52 upgrade. Multiplying that out, 16 terabytes of data have been sent to people who have upgraded their Az install, via BitTorrent.
How much would that have cost a company to send, just in bandwidth costs? I used a rough guess of $400/month for a T1, which would give you about 3.8TB in upload bandwidth (assuming 1.5MB/sec). In rough numbers, it would cost somebody over $1600 to send out that much data. Of course, using the costs of my DSL line or 56k modem, the costs would have been $12,000 or $25,000, respectively.
Now, in the case of a company, spending $1600 to keep 3.2 million customers happy is nothing. But for a successful Open Source project, run by volunteers, that's a chunk of change. Having a mechanism like BitTorrent to shoulder the expense of something like that is significant, especially again for a volunteer organization.
$1600 in 10 days. Pretty cool.