I currently use mediamonkey and doubletwist. Mediamonkey because it does conversion, and all my music is stored as flac and lossless wma, neither of which the pre can play. But the on the fly conversion it does takes a long long time if you are trying to move a ton of music. So I use media monkey to create a shadow mp3 library of all my music, and the doubletwist to sync that shadow library of mp3's to the Pre.

If media monkey would only use that mp3 library as cache for on the fly conversions, I could do it all with MM, but it doesn't.