I’m a regular user of Apple’s iTunes Remote application for iPhone. It lets me listen to my music upstairs while controlling my iTunes downstairs (I have speakers upstairs wired to the downstairs media center).
Now that I’m subscribing to Spotify and using it more as my primary media app, I’ve begun using SpotRemote. It’s essentially an iPhone and iPad-based app to let you control Spotify remotely with your mobile device. It’s $2.99 at the app store and seems to work well. You do need to download the SpotRemote server software from their website.