If you ever wanted to send a private email message to one of the person in an official email list which you are sending to a group of people, then Bccthis is the perfect plugin. Bccthis is an Outlook plug-in that lets you safely send personalized messages to email recipients along with your original e-mail. For example if you are sending an email to your boss and your colleague is in the cc list, then you can sent a private email with the original email such that it can be seen only by your colleague.
The Outlook plugin adds in a separate body and recipient field to a message. You can select the recipients who will be receiving the private message. Sadly this plugin works only with Outlook 2003 and 2007, hopefully they will be bring one for 2010 as well.
There is also a Twitter private message option for Firefox users. You can install this add-on for Firefox or use the bookmarklet on your browser. With this option, you can tweet normally and apart from that, you share an additional message with selected followers that links back to your public tweet.
All Bccth.is message are available on your dashboard so you can continue threaded conversations with the people. You can use this as a web based Twitter client as well.