WordPress.com this month launched SFTP and PHPMyAdmin access on Business and eCommerce plans. This enables publishers using WordPress.com to upload custom themes and plugins directly to the website and manage the database.
With SFTP access, publishers can directly upload the plugin files for more speed and troubleshooting, develop their own themes without having to download and reupload and have direct access to the media files.
WordPress.com also released a guide on how to use the SFTP.