The best replacement for greenhopper is rally.

The best alternatives to greenhopper are rated as following:
★★★★★  rally
★★★★  versionone
★★★  pivotal tracker
★★★  mingle
★★  ontime
★★  scrumworks
★  scrumdo
★  targetprocess
  icescrum

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

Other search terms related to greenhopper and their ratings are:
★★★★★ jira
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Google Trends

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greenhopper
rally
versionone