A WYSIWYG editor should be lightweight and easy to use, and should not break your design. That is what Expresso sets out to do. There is a choice of either a full, basic or light toolbar, although all toolbars only contain the most crucial text editing buttons. You can also create a completely custom toolbar. Strict precautions are in place so that pasting text from word or other sources won't produce ugly html and won't mess up your design when viewed on the site. You can define the css to be used within the editor so that the site's design is reflected there, making it easier to see the end result before publishing.
Expresso uses ExpressionEngine's native file manager so you will feel right at home when uploading and browsing files.
Expresso integrates with NavEE, Structure and Pages and adds links to your pages inside the link dialog. The link, images and other dialogs are also style to match the control panel theme.
Expresso is fully compatible with Matrix, Low Variables and Content Elements, so you can have full WYSIWYG functionality in your third-party fieldstypes.
Reviews taken from devot-ee