Skype (at least on my Mac) has a nice feature for editing my last message.
After sending a message I can press the up-arrow and Skype will pop up a dialog with my last message in it and allow me to edit it.
It also has a developer-centric way of editing using regular expression substitutions. Again, after sending a message like
se you at 10
you can type this as your next message to "fix" the typo: