Mac: Swifty is a menubar app for the Mac that makes sending private messages to your friends via Twitter, Facebook or Gmail super-fast. Just click the icon in the menubar, type your friend's name, the message, and press send.
The app lives in the menubar and runs quietly in the background until you need it. Just click the icon and start typing the name of the person you want to send a message to. You can send the same message to multiple people, but keep in mind that Swifty only sends private messages, meaning Twitter DMs, Gmail messages and Facebook messages -- not public status updates or tweets. Type in your message and hit send.
By default, emails and Facebook messages have a "Sent via Swifty" signature, but you can customise it to your liking. You can also add a keyboard shortcut to auto-open Swifty.
I've been testing Swifty for a little while, and it really is fast. It needs a lot of permissions to do what it does, so be ready for that -- it will want to see your contact list (to auto-fill your friends' names) and to post on your behalf (to send the messages you compose), but it won't post without you knowing, and you can revoke its permissions at any time. If you use Twitter DMs and Facebook messages in addition to Gmail all the time, it can save you from switching between tabs and apps. It's $1.99 in the Mac App Store.
Swifty ($1.99) [Mac App Store]