If you use iTunes, I recommend iTunes Agent by Jaran Nilsen.

It syncs whatever playlist you create under iTunes. And each playlist can be put in a specific folder on the N900.

The official build only lets you sync one playlist to each N900 and does not sync mp4. From sourceforge forum: "iTunes Agent uses the drive letter to manage a registry of the different devices, hence if you set up two configurations which will match the same drive letter, only one of them will actually be syncronized by iTunes Agent as it will only detect one of them when performing the synchronization."

With a little patching and code (thanks to crea-doo ), I added multiple playlist syncing and mp4 support.

I like having a song folder, audio podcast folder, and video podcast folder. PM me if you want the build which includes mp4 support and crea-doo's multiple playlist patch.