Recently I was working on my ESXi lab environment and I wanted to get ESXi to detect a Realtek onboard NIC.  There's nothing special about the servers, they were servers I put together from desktop components.  However, I did have some Broadcom PCI-E dual port nics in each server.  After ESXi 4.1 U1 was installed ESXi only picked up the 2 nics on the dual port card.  I wanted to get the onboard nic working also.  Offically the Realtek nics are not supported on the VMware HCL, but obviously for a lab/test environment, you may want to get them working.  Here's what I did to get the NIC to be detected by ESXi.

1.  Download a new oem.tgz file from Here (this file has the realtek 8111/816B driver in it)

2.  Browse your local datastore from your vSphere Client.

3.  Upload the oem.tgz from above to the datastore.

4.  Enable SSH on your ESXi server.

5.  SSH into your ESXi server and change directory to /vmfs/volumes/Hypervisor1.

6.  In that directory you'll see an oem.tgz.  Rename the file to oldoem.tgz. (e.g. mv oem.tgz oldoem.tgz)

7.  Path to where you uploaded the new oem.tgz to then copy it to the Hypervisor1 location. (e.g. cp oem.tgz /vmfs/volumes/Hypervisor1)

8.  Reboot your ESXi server and when it comes back up, the Realtek RTL 8111/8168B PCI Express Gigabit Ethernet Controller should appear in your list of network adapters.  Attached is a picture of it installed and working.