Just repeating myself here because I just shared my first “substantial” SmartApp and I used Cloud9 to do my editing. (The editor supports a long list of coding languages, including Groovy.)
Cloud9 provides a “workspace” (a virtual Linux session) with various tools including git (and it fully supports remote repos on GitHub; so I was able to do my git-ing very easily while on Cloud9.
Cloud9 will even SSH out to your own execution environments (ssh + Node.js required?), so integration with SmartThing’s simulator is not out of the realm of possibilities.
More general, their rich editor is Open-Source, “ACE”, (a successor of Mozilla Skywriter). At first glance, it seems quite “atom” like … I’m sure there are some detailed reviews and comparisons out there.
Atom or ACE … any Open-Source “full feature” code editor would be a great candidate for SmartThings IDE; and, a GitHub (or a SmartThings hosted git repo?) seems an obvious choice for code management and release management. Owners and contributors can be assigned to the repos for each SmartApp, and pushing for publication just requires merging into the “release” branch for the project. … Nice, no?