JRiver now has a new Playlist Server system, called Doctor Who.
You can upload your playlists and smartlists to it, then download them from another machine.
Here's the forum topic on Doctor Who: https://yabb.jriver.com/interact/index.php/topic,118655.0.html
You can browse other users playlists and download them.
To upload playlists
You will need to turn on the Doctor Who feature in Tools > Options > General > Features.
Then check for updates one more time. You should see Doctor Who at the bottom of your playlists. Drag any playlist to it to upload.
Click on the cloud icon next to any lists on the right to download.
Any downloads will be found at the top of playlists, under "Downloaded Playlists".
Before you can upload a playlist, you must sign up for an account using your forum name, and then use the e-mail you receive to activate the account