.RCPROJECT File Extension

Movie project created with iMovie '08 (a.k.a. iMovie 7) or later; includes a timeline with references to video clips, as well as effects, audio tracks, transitions, and other settings.

Unlike previous versions of iMovie, iMovie '08 saves all files by default in the ~/Movies/iMovie Projects folder. Each movie project in this folder will show up in the "Project Library" pane of the iMovie editing window. Since iMovie '08 saves video projects on the fly, there is no "Save" option in the File menu.

NOTE: You can import iMovie projects into Apple Final Cut Pro X by selecting File → Import → iMovie Project....