The best replacement for drools is jess.

The best alternatives to drools are rated as following:
★★★★★  jess
★★★★★  jrules
★★★★  ilog
★★★★  jbpm
★★★  openrules
★★★  esper
★★  odm
★★  activiti
★★  oracle business rules
★  pedigree
★  pega
★  royal canin
★  nutripet
★  brms
★  blaze advisor
  prolog
  tibco business events

Your alternative for drools is not listed? Suggest!
Speaking about drools, one probably shouldn't forget about jbpm.

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



Google Trends

Scale is based on the average worldwide traffic of drools in all years. Click on graph for more.
drools
jess
jrules