In the last years we have seen that Raspberry PI has got a great interest for the Open Source community and, indeed, it is endless the range of applications that RP (Raspberry PI) can be used due in particular for the size. After buying and using it for a while you realize that actually you need to do something more interesting than using it as a Desktop PC.
Moreover, I think it shouldn't be used as a Desktop PC because although RP has good performance considering the size (credit card size) it turns out that RP cannot execute heavy graphical processes.
The idea of create a Personal Server came into my mind several years before the origin of RP and the main purpose for creating a PS (Personal Server) is to serve only a really restricted number of users (usually only one) over the network and to be highly available whenever the users need it and, possibly, it should reduce the power consumption. As you can see Raspberry PI fits perfectly for this purpose.