The best replacement for jess is drools.

The best alternatives to jess are rated as following:
★★★★★  drools
★★★★  dean
★★★★  clips
★★★  logan
★★  jess's
★★  heather
★  jena
★  lisa
★  tess
★  taylor the voice
  jesse

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Speaking about jess, one probably shouldn't forget about guess.

Other search terms related to jess and their ratings are:
★★★★★ guess
rory
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Google Trends

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jess
drools
dean