I'm developing a client that uses Alfresco Web Services.
The next task that I have to face is deploy my own Web Service in Alfresco. I'm interested in extend Alfresco without the need of modify Alfresco sources.
It's not possible to extend Alfresco's SOAP Web Services without modifying Alfresco code - this is one of the reasons I would encourage the use of REST Web Services (Web Scripts - http://wiki.alfresco.com/wiki/Web_Scripts) instead.