BT Sync is now on WP8. Available here.

This app lets you have your own private cloud on your main computer, accessible from all your devices.

From the website -

BitTorrent Sync from BitTorrent that lets you sync and share an unlimited number of files and folders across all of your trusted devices. No file, photo, PDF is too big or small, Sync can handle any size and there are no limits. Sync all the GB you want and maximize your storage.

BitTorrent Sync for Windows Phones gives you access to all of your synced folders when you're on the go.

Features:
• Access all your Sync folders and files from your Windows Phone device.
• Add photos from your Windows Phone device to any of your Sync folders.
• Automatically back up your Camera Roll to a folder on your computer.
• File transfers are encrypted and protected by secure keys.
• Your information is never stored on a server in the cloud.
• Connecting the Sync application to other Sync folders is as easy as taking a picture of a QR code with your camera.