I'm going to check out the plugin API.
I'm glad you enjoy it, because you've produced really good work.
I'm now using MusicBee as my preferred music manager, for managing my music library, for listening to my music library, and for syncing with my Sansa Clip (MTP mode works flawlessly). MusicBee is fast and efficient, it's full featured, and has a really clean, intuitive, easy, and attractive interface.
I use MediaMonkey, which is a fantastic music manager/player/syncer, but it's interface is too cluttered for my tastes. Songbird looks nice and is full featured, but is slower than a snail. Windows Media Player is fast, looks nice, and syncs my Sansa Clip nicely, but it's library management and tag editing are less than satisfying. VLC is excellent, but lacks library management and syncing capabilities.
But MusicBee does it all well.
If I can find the time, I'm going to look into the C# version of the plugin API, and perhaps make an Amazon and/or eMusic plugin.