Former TUAW contributor Dan Pourhadi has become quite the iOS developer, and his OnCue app (US$2.99) keeps getting more features and popularity with every revision. The app was updated to version 5.01 this week, and has been burning up the iTunes App Store charts -- it's currently the #12 paid iPad music app and #24 on the iPhone.
In case you're not familiar with OnCue, it's an amazing mobile music player that gives users an easy way to create and arrange music queues. As an enhancement to your music library, it's a fun way to create queues on your mobile device with no need to resort to your Mac or PC and iTunes.
Some of my favorite features are the ability to create those queues with just a tap-hold-drag-drop gesture, and the advanced crossfade capability. Dan has provided crossfade to make transitions between songs without gaps, and you can even customize the "fade in from" and "fade out to" volumes if needed.
Another feature that I love is the ability to create Smart Queues. Like many of the smart folder features you find in other apps -- Mail.app is a good example -- you can create re-usable filters that use a set of rules to queue up songs. This replicates the capabilities of the Smart Playlists in iTunes, but lets you create the queues on your mobile device instead of on your Mac or PC.