I had never heard of Bccthis before they emailed us about their latest BlackBerry app. They have a relatively popular and well received Outlook plugin and have just released a BlackBerry client for it. The service allows you to attach private secure messages to select recipients in the To, CC, or Bcc fields while sending a message to a larger group of recipients.
Bccthis is useful since normally you would have to send a follow up email to the private recipients. No plugin is necessary for recipients to read the private messages which makes the service that much more useful. The app works with all types of email accounts from BIS to BES but is sadly limited to OS 4.6+ devices only.