Microsoft have recently announced the release of SharePoint Webhooks Developer Preview.
Webhook support in SharePoint provides an additional means to automate process in response to a change to or addition of a list item or document stored in SharePoint.
The mechanism allows you to register external application(s) with SharePoint online which will be called when an event occurs.
Historically many on premises SharePoint implementations used event receivers to run code in response to such events. Classic examples have included setting specific permissions on documents at point of creation, automatically creating folder structures, sending documents to record centres and so on. As event receivers are not permitted in SharePoint Online many organisations who have benefitted from using event receivers have been hesitant to migrate to the cloud. This announcement potentially eases migrations for such SharePoint users.