The best replacement for watin is selenium.

The best alternatives to watin are rated as following:
★★★★★  selenium
★  html agility pack
★  watir

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Speaking about watin, one probably shouldn't forget about specflow.

Other search terms related to watin and their ratings are:
★★★★★ specflow
You have in mind a companion for watin and it is not yet here? Propose!



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watin
selenium
html agility pack