Moodagent [iTunes Link] (free for now) puts a new spin on the iTunes Genius Playlist system. Instead of leaving it up to Apple to give you category based Genius Playlists, Moodagent, gives you some control over what songs will appear in a 25 song customizable playlist of your music.
You are given 5 colored bars, each with an icon representing an aspect of music. The symbols didn't make much sense to me, but by hitting an info button, I found that the icons represented, in order: Sensual, Tender, Joy, Aggressive and Tempo. I could have used a full explanation of what they meant by Aggressive and Tempo, but trial and error gave me a rough idea. By increasing or decreasing the size of each of the bars, a 25 song Playlist is created, complete with cover art icons, that can be played or saved for later recall. Increase or decrease one or more of the bars and the playlist instantly changes, so if you like a playlist, it's a good idea to save it before changing anything. Once saved, a tap will play your chosen playlist. When saving, you are also given the opportunity to title the playlist any way you like, with the first song being the default title.
Another way of making a playlist, again similar to the iTunes Genius playlist, is to select a seed song to base the list upon. Choose one song and then adjust the bars to tailor the list to your liking.
When first loading the app, you run a sync that indexes all the music on your iPhone/iPod touch running OS 3.0 or better, with their server. My iPhone has about 2100 songs on it and syncing took about 10 minutes. When done, I was told that it couldn't sync about 750 songs. This wasn't surprising since my tastes tend toward the arcane, but there's a solution for that. You are asked to download a Moodagent Profiler application that, as does iTunes Genius playlists, upload anonymous information from the iTunes library on your computer, which is assumed to have more music on it than does your mobile device. This information is used to expand the Moodagent database. After running the Moodagent Profiler on my roughly 6100 song Macbook Pro iTunes library, which took over an hour, I re-synced Moodagent and this time the number of songs that couldn't be synced reduced to 149.
So why didn't it find 149 songs?