UPDATE: Mozilla released Bespin today. Their post is at http://labs.mozilla.com/2009/02/introducing-bespin/.
I was at a conference today, and heard about a new software program or something that's coming out: Mozilla Bespin (pronounced like "best pin", but without the "t" at the end of "best"). It's essentially an online IDE, and a dang good one at that. From what was shown at the conference, it has integrated syntax highlighting and formatting.
Bespin has an integrated file explorer, used for exploring files within the project to be edited. It looks similar to Mac OS X's Finder when it's set to show each nested folder in a pane, with deeper levels to the right. (if someone knows mac's name for that type of view, post a comment and let me know.) You can open files and edit them, and then save them.
Bespin also has an integrated command line, which supports a lot of standard commands (such as rm and mkdir).
Bespin either has or will have support for integrating with version control systems. I can't remember if they said that it would integrate with CVS or Subversion, or both, but it was one of those.
Bespin is going to be officially released in a week or two, according to Mozilla, so let's see what happens. Until it officially comes out, take everything that I say here with a grain of salt, since I completely forgot a pen and paper (and my computer), so I couldn't take notes during the presentation.