Apple has a treat waiting for all you iPhone/iPod touch owners who also happen to be iTunes users (I imagine there is a high level of overlap between the two groups): Remote. Remote is a free application that turns your iPhone (or iPod touch) into a remote control for iTunes. Sounds simple, but once you use it you'll wonder how you did without it.
Apple's implementation makes it a fairly seamless experience. Your iPhone and Mac that hosts your iTunes library need to be on the same Wi-Fi network (nope, it doesn't work over EDGE or 3G), and you need to enter a code from your iPhone into iTunes but that's it. From there it feels like your entire iTunes library is now on your iPhone. The best part is that if you have multiple sets of AirTunes speakers, like I do, you can control where your music is routed to.
Did I mention that Remote is free? Yep, this great little app costs nothing.
Check out this gallery for a bunch of screenshots.