SubEthaEdit is a full-featured text editor and live collaboration platform with many specialized tools for software developers. Its streamlined interface focuses on speedy workflow--whether you're working solo or in tandem--and several users can edit a single document simultaneously using a simple, Bonjour-based system that lets you effectively drag and drop fellow collaborators into (and out of) docs, without any cumbersome setup.
SubEthaEdit also puts a premium on customization, with detailed choices in its preferences, good interoperability with most third-party apps you're likely to be using (like FTP clients and IDEs), and support for AppleScript and multiple coding languages (including important extras like language-specific regex searching and new code folding features). While SubEthaEdit is primarily aimed at developers, many users have had success with this app in less technical environments, such as collaborative writing and note-taking. You can find other text editors with similar code-editing features for less, but SubEthaEdit stands out for its elegant interface and easy-to-use real-time collaboration and sharing capabilities.