QuickTime, and what a Mac or an iPhone records into. Structurally it's almost the same file as an MP4 - both grew from the same specification - but plenty of software sees the extension and refuses anyway, which is the entire reason MOV to MP4 is a conversion people need.

Every conversion below runs in this page, on your own machine. Nothing is uploaded, there is no queue, and there is no account to make. Pick a target format, or just drop a MOV file above and choose afterwards.