For around $200 a year, VMUG Advantage provides members with 365-day evaluation licenses for nearly all VMware products. While they focus on current versions, it is the best way to stay within the legal ecosystem while learning.
Several public repositories host "curated" lists of license keys for older VMware products. esxi 55 license key github hot
For a spare laptop or an old PowerEdge server, ESXi 5.5 might be the only option. For around $200 a year, VMUG Advantage provides
However, GitHub is incredibly useful for legitimate ESXi 5.5 automation and enhancement. Here are ethical GitHub projects relevant to ESXi 5.5: For around $200 a year