I built a VMware ESXi server a couple months ago for very little money with used parts. It’s been running for the last couple months, is fast enough for me and houses systems I use every day.The biggest caveat: It’s 32 bit only. If you need 64bit systems this won’t work for you.
Part list:
ebay (used equipment bids)
HDAMA motherboard including dual Opteron 2xx CPU, PSU $40 shipped
8GB DDR 2100 ECC RAM: $100 shipped
NCIX.com/newegg
500 GB SATA Hard disk: $50 on sale
Dynatron A43/A43G 70mm 2 Ball CPU Cooler: ~$35 for two shipped
Syba SD-SATA-150R 2 Channel SATA Controller Card (Part#: SD-SATA150R): $15
The last part was the really necessary part. ESXi is very difficult to install on anything but hardware RAID controllers which can be very expensive. According to the ESXi Whitebox HCL it supported a very inexpensive chipset, the Silicon Image Sil3512. The SD-SATA150R card I bought had a Sil3512 chip. There are probably many others on the market but this one works for sure.
The Dyantron heatsinks are very quiet and cool the CPU well. The HDAMA motherboard is not the best but it was used in some now defunct blade servers which makes them very cheap. I actually bought two shipped for $40 on ebay. The opteron 2xx CPUs are good. For 64bit emulation, you need something in the 2xxx range of opteron but look at about $400-500 for one of those systems.
ESXi 4.0 installs without any problems, no tinkering required on this system and it hasn’t crashed in two months.