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.

Free for Outlook, Gmail and Twitter.

Link: bccthis