Webhooks are user-defined HTTP callbacks that allow interactions between otherwise independent web applications.
Thorn is a Python framework for building webhooks and event-driven applications. There is also an introduction post with more information on using Thorn.
PyWebhooks is a proof-of-concept library for building webhooks-based services.
Webhooks Provide an Efficient Alternative to API Polling is a high-level overview of the advantages of webhooks over alternatives such as constant polling for updates.