Unique is a library for writing single instance application. If you launch a 
single instance application twice, the second instance will either just quit 
or will send a message to the running instance.

Unique makes it easy to write this kind of applications, by providing a base 
class, taking care of all the IPC machinery needed to send messages to a 
running instance, and also handling the startup notification side.

Unique aims to replace the BaconMessageConnection code that has been copied 
by many projects and the code using Bonobo and D-Bus.

WWW: http://live.gnome.org/LibUnique