Windows/Mac: We're fans of Hemingway because it makes it easy to glance at a block of text and edit it by highlighting all your syntax. Now, the webapp is available for Windows and Mac.
The desktop version is essentially a skin of the web app, but it works offline and it seems to handle larger blocks of texts better. The desktop version does have a few extra tricks up its sleeve, including support for Markdown, a more useful "Write" function of the app, and the ability to save. Otherwise, it's still a handy tool to proofread short blocks of texts like emails.