The omake build system has the awesome feature called "server mode", invoked with the command line switch "omake -P". This will cause omake to monitor the file system for any file changes -- when there is a file change it will launch the appropriate build action.
Unfortunately omake does not seem to be actively maintained anymore and the -P feature has severe problems on Ubuntu 12.04. (I can discuss these in some detail if anyone cares).
Does anyone know of a build system like make/omake that has a feature similar to omake -P?