This has to be one of the best End User License Agreement clauses ever:
4.1 Permission to utilize Your computer. In order to receive the benefits provided by the Skype Software, you hereby grant permission for the Skype Software to utilize the processor and bandwidth of Your computer for the limited purpose of facilitating the communication between You and other Skype Software users.
Lots of searching on the web yields very little about Skype that isn’t “it’s the greatest thing ever”, but there is this blog post where the author talks about the fact that Skype makes open hosts on the Internet into “supernodes” to facilitate conversations between computers behind firewalls and NAT devices. Smells a little fishy to me.