Just released today, ZumoCast (free) sounds like a great app. It lets you stream your videos and music that are on your computer to your iPhone or iPad wirelessly via 3G or Wi-Fi. No syncing necessary. It automatically converts the streaming content on-the-fly to a format appropriate for your device. One advantage of this is that because your files stay on your computer, you can avoid filling up the memory of your device. The app also lets you save music and video for offline viewing or listening.