Bccthis is a free tool that allows a user to append a private note for a subset of recipients in a group email without having to compose a second message. The private note is sent only to the intended recipients; the rest of the group cannot view it.
Bccthis is available as a plug-in for Microsoft Outlook and as an addon for Firefox that hooks into Gmail and lets you send a secondary email to select contacts without leaving the composition window.
The Firefox addon attaches an expandable Bccthis compose box below Gmail's main composition window. This is where you type your secret message and choose recipients. The Outlook version of Bccthis offers more customization and formatting options, but the mechanics work the same way on Firefox.
The secret message appears in a box within the body of the main message as shown below.
The Outlook version can consolidate multiple incoming emails if the sender is also using Bccthis for Outlook. Else they end up receiving two emails.
Bccthis is also available as a web based Twitter client that allows you to send a separate discreet message to selected followers and continue that discussion in private. Bccthis is a free full fledged Twitter client that offers you all the features and functionality that you've come to expect, like threaded conversation and built-in URL shortener.
Bccthis also has a Blackberry app.