The best replacement for isight is facetime.

The best alternatives to isight are rated as following:
★★★★★  facetime
★★★★  facetime camera
★★★  facetime hd camera
★★★  facetime hd
★★  motion eye
★★  logitech c920
★  pureview
★  back camera
★  face time
  facetime cameras
  quickcam

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Speaking about isight, one probably shouldn't forget about omegle.

Other search terms related to isight and their ratings are:
★★★★★ omegle
★★★ imovie
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Google Trends

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isight
facetime
facetime camera