Scratch

http://scratch.mit.edu/projects/ScratchExpert/1945657

This is a simple project that can be imported into any two-player mesh (link to wiki) project so that two players can chat during a game. Here are some features:

-Asks for a display name at the beginning of the project.
-Press T to talk (can be changed)
-Up to 3 messages shown at once (appears to scroll).
-The chat can be moved to the top, middle, or bottom of the screen.
-The text's appearence can be customized, such as  Hello! instead of Red: Hello! (Red as an example player).
-The length of messages can be limited (not recommended though) (recommended length is 20-30 characters.
-You can have your project add messages to the chat.

I've tried my best to make it as easy as possible within the project to customize (with the help of comments and such).

Also some credit to RHY3756547 for the basic engine in his project, "LINE Mesh Co-op" (link to project).

In the future I may add a list-based version, a version for more than 2 players, and a version for non-multiplayer projects.

Enjoy.