Good companion app to Apples own Remote App. The Remote App gives you access to your iTunes library so you can play your music to powered speakers or stereo system anywhere in your house connected to an Airport Express in your wi-fi network. But if your Mac is asleep and/or iTunes is not open youre out of luck. The Control! app not only wakes up your Mac remotely but also lets you open iTunes. After youre done you can quit iTunes and put your Mac to sleep again. This functionality probably should have been built-in into the Remote app but Control! works perfectly plus it has other good features as well.
Update
Does not "work perfectly". After using it for sometime I find it is good enough but the connection is unreliable. Forced to keep iTunes open, otherwise Remote could not access it after waking the computer up. The interface could use a little improvement too. 3 stars for now.
Jospal217 about Control! Mac - Remote Control, File Browsing and Video Streaming for Macintosh