Firefox: Ever wanted to send a message to a bunch of people, but with a note tacked on the end for just one of them? Firefox extension Bccthis adds this feature to Gmail, saving you from writing multiple messages.
The extension adds a second collapsible text box to the bottom of Gmail's compose view, allowing you to add another message that can be sent to any number of the message's recipients in the to, cc or bcc section. Gmail will then send two copies of the message: the original message, and an additional message to your Bccthis recipients with your secret message, so you don't have to compose two yourself. Plus, for messages where you don't need the extension, you can easily minimise the text box so it doesn't take up so much space. It's very useful for prompting follow-ups from one co-worker, tacking on impromptu "love you" post scripts to your significant other or planning mutinies.
Bccthis for Gmail is a free download, works wherever Firefox does.