Note: Actual checksums are published in VMware KB articles (e.g., KB 2096145). Always cross-reference.
Maintains a download landing page for ESXi 5.5 Update 3a. vmware esxi 5.5 iso download
If the hash doesn’t match a known official source, . Note: Actual checksums are published in VMware KB
Several archival sites host ESXi 5.5 ISOs. vmware esxi 5.5 iso download
If you download from any source, always verify the checksum to ensure the ISO has not been tampered with.
: While a free version existed, VMware has recently ended the general availability of the free "vSphere Hypervisor" for newer releases. Spiceworks Community Performance & Feature Review