This is an excellent course in Katharevousa, and is what my teacher and I are using as our main text. It is entirely in Greek, and was designed for children, so the language is easy to understand. The main site contains many other schoolbooks with varying subject matter, and the search features are quite advanced. For those, like me, who use a screen reader, all of these are in photo-scanned pdfs, so cannot be read without sighted assistance. My teacher writes out the lessons and sends them to me as rtfs, since the polytonic system made them virtually impossible to scan with ocr software. We are also using "Γραμματικὴ Τῆς Νέας Ἑλληνικής Γλῶσσης ((Τῆς Ἁπλῆς Καθαρευούσης)" by Ἀχιλλέας Τζάρτζανος (not available at the below site) as supplementary material.
http://e-library.iep.edu.gr/iep/collect ... 112&tab=01EasyAccent is the program that I use to write in polytonic. I am providing both the original site as well as another one where I located the file. The original no longer works, but perhaps, it can be accessed via the Way Back machine at archive.org, which is not working with my screen reader today. For those new to polytonic, they mention Biblical (Coine) Greek, but that can be ignored for those of us learning a modern form and still using the full system. I am not sure if the version at Software Informer is the latest one.
http://www.biblicalgreek.org/links/fonts/keyboard.phphttp://greek-polytonic-easy-accent.soft ... ormer.com/