Chrome: If you love keeping your workflow within the browser, Textdown is a surprisingly powerful text editor that runs completely inside a Chrome tab.
Textdown is a plain text editor by trade, but it supports the Markdown markup language to create links, lists and rich text. (If you aren't up to speed on Markdown, we have a great guide to get you started.) Just open a new tab in Chrome, and click the Textdown icon to open a new document. You can name it what you want, then start typing immediately.
You can write out your Markdown syntax by hand, but the app also includes a lot of standard keyboard shortcuts to speed up your workflow, or help you along if you can't remember how to format something correctly. Control-D will open up a handy cheat sheet of keyboard shortcuts, and Control-> lets you configure settings and create custom text expander macros.
The concept is similar to previously-mentioned Hashify, but with a lot more options, including the ability to use the app offline and save files locally to your desktop, either as plain text or as HTML. Unfortunately, Textdown doesn't autosave, and doesn't have any cloud storage, so be sure you save locally before closing the tab (if you screw up, File > Reopen Closed Tab will reload your work). If you want to start working on the same document later, Textdown can import any text file from your hard drive with ease.
For a webapp, Textdown holds its own against native Mac and Windows Markdown editors, and it's definitely worth a look if you like to live inside the browser.