What is the difference between ESX 3.5 and 3i? Foundation, Standard and Enterprise?

There are actually three different versions, namely ESX 3.5 ESXi Embedded and ESXi Installable. ESXi used to be called 3i.

Differences between ESX and ESXi are listed in  http://kb.vmware.com/kb/1006543.

3.5 is the "Full" traditional version which is installed from CDROM usually onto local hard drives of a server (boot from SAN is also possible). It comes with Service Console, which is a Linux based administrative virtual machine.

ESXI is a newer mall-footprint hypervisor. It comes preinstalled with the server hardware BIOS (Embedded version) or you can install it yourself for instance on a flash drive (Installable version). Alone it needs additional software to manage it.

3.5 and ESXi have quite similar hardware requirements and same supported hardware vendors. ESXi does not yet have wide support on as many servers as 3.5. Details can be found in the HCL: http://www.vmware.com/pdf/vi35_systems_guide.pdf

Comparison of the different editions (Foundation, Standard, Enterprise) ions is available at: http://www.vmware.com/products/vi/esx/esx_buy.html