What format does Google Chrome Developer Tools save data as?

The HTTP Archive format (HAR) format is intended to be a format for sharing HTTP data across tools. Although there are a bunch of tools that support HAR & allow exporting, only a few among those appear to allow import:

• HAR Viewer (offline, online)
• PCAP Web Performance Analyzer
• Fiddler
• Charles
• HTTPWatch
• HAR Analyzer in Google Apps Toolbox

.har seems to be an HTTP Archive Spec file. You can use HTTP Watch to open such files. FWIW those are JSON files

Starting in Chrome 62 you can simply:

Drag and drop a HAR file into the Network panel to import it.

(From the release notes.)