Starseige:Tribes - Typing in spare time on a ballistic trajectory... Though a good deal of that was also tapping out 3-4 keystrokes for the "V" communication tree, something that I think a lot of games could benefit from even in this era where there's enough bandwidth for voice chat.

With the amazing amount of client-side scripting the game supported, people made their own variations on the communications system, even carefully playing audio clips over one-another to get new speech.