SaaS rather than (P)latform, requires your own AWS.
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Pro Everything is supported
All languages, databases, and command-line tools are supported. Most machines are already set up with web project basics like Apache, PHP, MySQL, Ruby, Node.js, Perl, and Python and anything else can be installed via the SSH root access.
Pro Ability to signup with Github
And link account to oDesk and Facebook.
Pro Real-time collaboration
Real-time code and terminal collaboration with integrated chat abilities.
Pro Built-in terminal
With 256-color support.
Pro Built-in package manager
The Koding Package Manager (a successor to Apps) is command line tool for installing packages like nginx, Redis, VNC, Wordpress and many others. It consists of officially supported and user-built packages.
Ability to create different sized communities that can be public or private.
Pro Various file upload options
Koding supports drag & drop, Dropbox, clone from Github, FTP and the ability to access them using SSH.
Pro Great community
Koding has a community feature that makes it easy to collaborate and share ideas with like minded developers all over the world. And the community itself is very active and helpful community.
Pro Unlimited domains and subdomains
Unlimited user-defined domains and subdomains for private or shared VMs can be set up.
Pro Social Stream
There's a built-in social network with developers from all over the world.
Pro Capable editor
Koding uses the Ace editor that's developed by Cloud9. Besides the basics, it covers most important advanced code editor features such as code folding, converting cases, auto-completion, code analysis and refactoring, regex search and offers easy access to relevant documentation.
It also gives access to the CLI, has support for Vim and Emacs keybindings, includes multiple cursors and zen coding mode that removes all distractions and allows focusing on code.