Hi there!
What do you mean by outside program? If you wish to edit HTML, and the visual layout of your site, C-Panel includes one, just click "Edit HTML" in the filemanager.
As far as software for your computer goes, there is always Dreamweaver (really expensive, so maybe not so good), or a few other free alternitives, Notepad will always do, or there is Coffee Cup HTML Editor, they have a free version (
http://www.coffeecup.com/free-editor/) or there is a full (paid, but cheaper than Dw) version, that will include most of Dreamweavers features.