[knute]

There was a mode in ICQ long ago where the person you were messaging could see everything you typed, as you typed it. A friend of mine always wanted to use it and it was mildly terrifying.

[progval]

Google Wave did this too, but it was enabled by default. It sure takes a while to get used to it.

[hoytech]

Also unix talk(1): https://en.wikipedia.org/wiki/Talk_(software)