Not Safari, the excellent OS X web browser, but the O’Reilly Network Safari Bookshelf.
It’s really a NetFlix for the computer development world, only the content is fully searchable and available online. I’m currently using my free preview, and it’s so affordable, I really can’t imagine any professional developer wanting to do without it.
Kudos to Cobbie Behren, who was nice enough to respond to a comment I made on his blog by beginning a very useful conversation! Thanks Cobbie!