How can I download entire folders/albums or songs from my PC (over wifi, BT, or USB) to my phone to take on the go I have tens of thousands of songs on my PC organized by artist/album in folders, and would like to be able to view my collection and manage what comes with me when I travel. Ideally I could browse and select what to download to the device over wifi/BT/cable, with some sort of server app running on my PC.

Bonus points for some sort of "sync" type option where it maintains the directory of music that is on the PC (even when offline) so I can queue it for download as I remember music during my travels, then have it download it when I get home.  I'm not necessarily looking for it to be a music player - just something to manage my 250GB collection between my phone and computer.

I'm NOT looking for a streaming app, but for downloading. In fact, it doesn't really have to just be music - it could be movies, or even docs I guess...
Best variant, combining most of your wishes: FolderSync. I'm using it myself (Pro version after having tested it one day), and I'm very fond of it. You can setup multiple folder pairs (which folder on your device should be synchronized with which folder on the "other end") for multiple "accounts", where an "account" simply identifies "the other end". That can be a cloud service such as Dropbox, but as well your Windows PC via Samba (Windows Share), or your Linux machine via SFTP (SSH), some FTP server, ... FolderSync supports a lot of protocols.

Moreover, the full version has support for tasker, giving you a load of options for when to sync. I use it e.g. to sync my backups: "when charging, and it's 4am, (optionally more conditions), then call the task named 'SyncBackups'" (the latter then tells FolderSync to sync multiple backup locations with my Linux machine). Well, if you don't have Tasker: FolderSync also has a time-based scheduler included.



Remark: My suggestion lacks the "remote management" on your PC. I'm not aware of any all-in solution here, especially for "offline queuing". When you're at your PC, that's not a big deal: simply copy the files to your sync location on the PC's end. For other cases, I might be able to add some "online management" using your data connection (or WiFi hotspot), depending on the OS your PC is running.