The best replacement for sun cluster is veritas cluster.

The best alternatives to sun cluster are rated as following:
★★★★★  veritas cluster
★★  vcs
★  oracle clusterware
★  veritas cluster server

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Speaking about sun cluster, one probably shouldn't forget about oracle rac.

Other search terms related to sun cluster and their ratings are:
★★★★★ oracle rac
★ asm
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