[August 2018 Update]
Feel free to user S2ajax if you are looking for something lightweight.
On the other hand, between jQuery 3, React, Angular, etc. S2’s concept is definitely getting quite long in the tooth.
S2ajax is a ‘managed’ AJAX framework. It is derived from Sajax, written by the Modern Method folks a few years ago.
This modern implementation allows the developer to declare classes on the server and instantiate them on demand on the client.
S2ajax is available under the very permissive BSD license.