Tropo is a cool and relatively new service for on-line voice application programming. As described in this article, Tropo generates phone numbers in +990 range, and calls to these numbers from Skype are free of charge.
Tropo provides an API for many programming languages, and quite interesting features. Also it can host your application source, so you don;t need any additional services to run your application.
Note that it takes about 5 minutes for the Skype number to become active after creating a new application.
That’s it, and you can create an application with this file. Tropo automatically assigns a few new numbers, including a Skype Voice number for free calls (+990009369996210741 in my case. You would hear MOH from my FreeSWITCH server).
UPD: this feature appears to be known since long time via the Voxeo service, and Tropo (being a service on top of Voxeo) makes it easier to use.