Original implementation of Javascript
mxr.mozilla.org
mxr.mozilla.org
The original implementation was written in C by Brendan Eich, and shipped in Netscape 2. The code currently used in Mozilla Firefox is a direct descendant of that.
And looking at it, I'm still not sure it isn't the original (or rather, early descendant of). Late 2005 is only the license change; the first revision I see (in this folder) is 1999-01-29, and the first version of some files says "initial checkin for source reorg" (remember CVS?).