Codebender is an online platform for wiriting sketches and uploading them to the Arduino-compatible board of your choice. It's like having a cloud based IDE for writing and debugging as well as having a cloud storage to keep and share your sketches from one place.
Like any cloud service, Codebener is great if you jump between different machines, but I think that the "killer-app" for this service is in Education. Teachers and workshop leaders can prepare examples or code outlines in advance and share them with your group.
The setup process is very simple, and the developers have created a nice walkthrough to help you get up and running as quickly as possible. You can view their steps here.
In order to successfully use Codebender, make sure that you have Chrome installed.