Notepad

Chat was the first thing to follow after the creation of the email protocols and clients. Originally there was IRC and then that expanded.

There is now IRC OFTC (The Open and Free Technology Community) as well, aka WebChat
Linen is an interesting alternative to chat which incorporates both Discord and Slack messages and also has forum features.
Slack
Discord
Google Hangouts
Whatsapp/Facebook messenger
Apple iMessage
SMS for texting
Signal
Telegram
Line

Forums: