Creating personalized playlists for your iPhone is great if you're a control freak with plenty of time, but I've increasingly come to rely on the Genius function introduced with iTunes 8. But Genius requires you to build playlists around a single "seed" song, and is often weak on variety--it almost always picks two or three other songs from the same album as your seed song, and its other choices tend to run to the same era and genre.
Moodagent, a free app for the iPhone and iPod Touch released in December, offers a more innovative take on automated playlist creation. It gives you five sliders that let you set desired levels of sensuality, tenderness, joy, aggression, and speed. These categories might seem kind of arbitrary, but in practice the resulting playlists make a lot of sense: turning the sensuality slider up gave me lots of slow and funky stuff like Beastie Boys instrumentals and Portishead, while turning aggression and tempo all the way up returned lots of material from the Reverend Horton Heat, Pixies, and Dead Kennedys. Best of all, the playlist adjusts itself on the fly when you change the sliders.
Like Genius, the Moodagent playlists are 25 songs long and tend to contain a few songs from the same album. But beyond those same-album tracks, it seems to offer quite a bit more variety in terms of year and genre. You can also pick a seed song to create a playlist if you like, but it's not required. If you come upon a winning playlist, you can save it to the app.
Moodagent requires an active Internet connection to analyze the songs on your device against the 8 million songs in its database. It took about 10 minutes to get through 3,000 songs for me, but about 1,000 songs weren't recognized, or were protected by DRM. (Moodagent won't work on DRM-protected tracks.) You can download a free desktop application that transmits non-personally identifiable information about your iTunes library to the company, which in the long run should help Moodagent recognize more of your songs. But the process was a bit weird--when I launched the app, it simply opened Internet Explorer for a half second, then shut it back down. I have no idea whether the desktop app worked or not.
This minor glitch aside, Moodagent is a worthwhile download if you're looking for new ways to listen to your music.