The best replacement for nutch is solr.

The best alternatives to nutch are rated as following:
★★★★★  solr
★★★★  heritrix
★★★★  lucene
★★★  scrapy
★  crawler4j
★  bixo
★  crawl anywhere
  manifoldcf

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

Other search terms related to nutch and their ratings are:
★★★★★ lucene
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Google Trends

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nutch
solr
heritrix