[nelsonic]

View Source has been available since "The Beginning"; its how we learned html back in the day. As @Aredridel says, (the grandfather of all browsers) Mosaic had it. And since Internet Explorer was based on Mosaic, IE had it View Source from the start too.

Here's a good history lesson on "The Birth of Mosaic": http://www.nsf.gov/news/special_reports/nsf-net/bina.jsp