Is there an OS X equivalent to iOS's Remote App?
iTunes Remote Control is probably the easiest, but is no longer updated, and I've heard reports of some issues with it under Snow Leopard. YMMV.

Edit: One other major option would be to control iTunes from your browser. A variety of commercial and free solutions exist to do this. This is one of the more popular DIY guides on the subject. Playerpal and PatioTunes are two examples of commercial variants on the idea. N.B., I've used neither, and can't speak to the quality of either app. Feel free to google around for more options on this front.