One solution to this is using email, but it can be hard to organize responses in the proper order, especially if there's more than just one other person involved in the conversation. Discusionus, on the other hand, is a simple application that uses your email as an IM service, so the whole conversation is grouped in an organized way.
You can receive your emails on the application while also getting rid of spam, or create a conversation between several users who have the application installed on their devices. Its simplicity makes it a great alternative to other traditional tools. Since it doesn't require any registration, you can just use your usual email account.