The Force.com IDE, an Eclipse plugin, is still by far the most popular route for developers to work on the Salesforce1 Platform - even as alternate routes like the internal Developer Console and MavensMate become more and more mature.
However, a common question about the IDE was always one of maintainability. Can it be maintained faster? Can new features be added? What if Salesforce drops support for it (which, for the record, would be extremely unlikely)?
Well to help out all of those questions, the Force.com IDE just went recently open-sourced, and you can access the repo on Github to see how it works, modify your own version, or submit new features as pull requests. I know this has been a goal of the Apex and Tooling teams for some time, and it is excellent to see it come to light.