I would recommend the Apple Remote app for iPhone. Pretty useful to use with your Mac.
And it's a must-have for AppleTV owners.
Also great for owners of Airport Express/Extreme. I use the remote on my iPhone downstairs to control iTunes on my Mac (which is upstairs), which then sends the music to Airport Express downstairs, which controls the sound system, which impresses guests. Should also work with PC (as long as you have Airport, Time Capsule, or something).