Other ways may include IP protocols, data-file manipulation, or signals. Signals are your friends. I'm not really sure of the exact mechanism, but i have never heard a Unix signal. I don't think my example 'hears' them either. The great thing about signals, is that all processes have to respond to signals.
Semaphores are cool.
They are like system-level arrays where you can lock a record. The best part is that the inner workings are black boxed. Ergo, little problematic conditions like race and corruption don't exist. Honestly, I'm winging it, consult a book for specifics. Lets build some requirements for our daemon.