Textlicious

Textlicious is a tiny service that lets you enter styled text in places that normally accept only plain text.

This way, you can make your tweets more expressive, your texts stand out, and your posts envied by all of your friends. You just need to highlight the text you’re typing in your app, right-click, and select the style you want from the Services pop-up menu. The available styles include bold, italic, serifed, typewriter, script, double-struck, and more.