I have used http://shiftedit.net/ and it seems to work pretty well. You can (s)ftp the files down and back and then edit them. I have only used it a few times for some PHP projects, but it has not failed me.
Hey there, great question. As /u/actioncamus mentioned, there is an active community over at /r/ChromeOS that specializes in this stuff. Additionally, /r/ChromeApps might be a good resource for you as well.
Since Chrome OS is a cloud centric operating system, there are very few tools for offline usage currently available, and even less that are designed for development. The majority of developers using Chrome OS have chosen to adapt to a Cloud IDE platform. It handles the FTP connection, the editing, etc.
The most popular Cloud IDE's are:
I'm personally a fan of C9 for my development needs.
Here's some discussion and options if you want to use linode/ec2 instances:
http://news.ycombinator.com/item?id=4083175
Ideally I'd want to have both local and remote access so a dropped connection wouldn't be an obstacle to work. That might still require a laptop with a standard OS at the moment though.