The best replacement for documentum is sharepoint.

The best alternatives to documentum are rated as following:
★★★★★  sharepoint
★★★★★  livelink
★★★★  filenet
★★★  alfresco
★★  opentext
★★  sharepoint 2013
★  adobe cq5
★  interwoven
★  oracle ucm
★  laserfiche
★  sharepoint 2010
★  docushare
★  onbase
  sap dms
  moss
  stellent
  hummingbird
  vignette
  open text

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Speaking about documentum, one probably shouldn't forget about sharepoint.

Other search terms related to documentum and their ratings are:
★★★★★ sharepoint
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Google Trends

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documentum
sharepoint
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