Sentence:  Since the launch early last year of [...] two Silicon Valley start-ups offering free education through MOOCs, massive <WSD>open</WSD> online courses, the ivory towers of academia have been shaken to their foundations. University brands built in some cases over centuries have been forced to contemplate the possibility that information technology will rapidly make their existing business model obsolete. 
Meaning of the word marked with <WSD> tags:  Not closed; accessible; unimpeded.