There are a number of useful software solutions for teaching online.
I am not sure what you mean by teaching writing. If you mean for kids just learning to write, then I'd have to say classroom lessons would be the best way to go. However, you can teach typing (software package plus conferencing).
If your learners are adult and have the basics of writing down already, you can have them do writing assignments and share/ collaborate with other students using something like
http://moodle.org/, an excellent, open source, php based course management package. I'm starting to use it with some of my university classes and the response has been very positive.
For all ages, you can get them involed in a key-pals program. Here is one that I have used:
http://www.epals.com/. Participating schools and classes all have various interesting ways of using the program and you can get yiour own students involved.