The best replacement for esxi 5 is xenserver 6.

The best alternatives to esxi 5 are rated as following:
★★★★★  xenserver 6
★★★★★  hyper v

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Speaking about esxi 5, one probably shouldn't forget about freenas.

Other search terms related to esxi 5 and their ratings are:
★★★★★ freenas
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esxi 5
xenserver 6
hyper v