OnCue is one of those iPhone apps you don't know you need until you start playing with it. The concept is simple: Apple's default iPod app allows you to import playlists from iTunes and customize the way you listen to music by combining different songs and artists in a single list, it lets you shuffle your music, but it's doesn't come with any queue functionality.
While you're listening to music on your iPhone or iPod touch and you're on the go, it'd be nice to be able to select the songs you want to listen to without having to pull out the device from your pockets every time. Use a playlist, you might suggest. But what if I don't want to create a playlist for each day? I just want to say "hey, today I want to listen to these songs in this specific order". You can't create a new playlist every day, yet you know what songs you want. You need a queue function. OnCue does just that, and it works with both songs and podcasts.
I haven't tested the podcast feature because I'm not a big podcast guy, but I see the app allows you to remember exactly where you left off, add bookmarks and even preview bookmarks in podcasts before you save them. Sounds nice. I've been testing the app as an alternative to iPod.app for a day though, and it works really well. Basically, OnCue mimics the appearance of the iPod application, only it adds a few shortcuts to build a queue of songs you can manage, edit and access at any time. While I don't get the color labeling options (you can assign a color to a song), I definitely appreciate the fact that with a few taps you can create a queue that, by the way, can be cleared at any time. You can even "save" a queue for future usage and scrobble everything to Last.fm.
To add songs to the queue, head over the Artists or Songs tabs and start tapping on the button next to each song. The button can be customized to become a "remove from queue one" or "send to the end of queue". By default, it's a CueNext button. Once the queue is built, you can out the iPhone away and just listen; alternatively, you can open the Queue tab and re-arrange songs to your liking by dragging them, or swiping on them to bring up a Tweetie-like menu. One feature I love about this app: if you hold the iPhone in landscape mode, it brings up a custom Cover Flow view that displays albums, Now Playing info and the actual queue as a transparent overlay.
Overall, OnCue provides an interesting, useful alternative to the iPod app if you've been looking for this very specific functionality to land on iOS. If you want to create music queues on the iPhone, this is the app to buy. No excuses.