Athento (based on Nuxeo) follows standards such as CMIS that allows it to communicate with other ECM platforms. https://doc.nuxeo.com/nxdoc/cmis/
It also provides of a powerful API very easy to use. You can see some usage examples from here: https://athento.atlassian.net/wiki/display/AEGSE/Playing+with+the+Athento+ECM+API
Athento also provides embeddables or scripts that allow you to embed some features in third party applications.