There is many others wireless technologies, like Bluetooth, WiMAX, Laser and even Infrared.
The answer will depend on how far are the server, throughput need, etc...
Laser is probably the most reliable but need no obstacle between both server (or at least laser endpoint) and can be quite hard to install. This is rarely used because it's hard to setup and can only be used in few case.
WiFi or WiMAX is I guess the best option. WiMAX will have the advantage that you have much less risk of interference with other network around but it's slower than wifi. If you have not tons of Wifi networks around WiFi is probably the best option, I would use 802.11n in the 5Ghz band (there is generally less interference in the 5Ghz band)
But you have to look what would be the best in your environment, 2.4ghz can work better than 5ghz depending of the environment.
Bluetooth slow and need server to be close, in this case you would use wire..
Infrared is the worst of Laser and Bluetooth ;)
Finally, take care that wireless is not very reliable, connecting servers with wireless is a bad idea and you must only do this if you have no way at all to use wire.
If you give us more information on what you do that and the environment (is server in 2 different floor, 2 different building, how far away, ..) we could give you a most better answer.
Edit following question update :
Optical link (Laser, etc.) will have the lowest latency (could even be better than Ethernet) but in your case this is probably not an option.
Electromagnetic link (WiFi, WiMax, etc.) will depend of Signal-to-noise ratio (SNR), in best case you will get around 3ms. SNR will become worst with distance because Signal become less strong. Noise is the result of interference so you can't control it as you would. With a bad SNR you will have packet loss causing retransmission (and therefore high latency). Also take care that WiFi can have a very high jitter
In you case you might be able to use power lines as suggested by chris_l, latency will depend of the environment, so it has to be tested but jitter is quite low in most case
On each ESXi, create a new virtual switch based on the direct connect NICs.
Using the 'Configuration' tab in vCenter on the ESXi, select 'Networking' and 'Add Networking...' Create new 'Virtual Machine' connections. Create a new virtual switch, and associate that switch with the proper NIC. Give the switch a meaningful name -- you'll need it later.
Now, in each VM that needs to talk to over the direct connection, create a new NIC (or modify an existing one), and set it's 'network label' the the switch you just created.
Assign the private IPs to those virtual NICs. Programs running in the VMs that use the private IPs you create will be routed across the direct connect.
Best Answer
Well... you would need to connect all of the servers to an appropriately sized network switch. They've been in use for decades. The first commercial multiport Ethernet network switch was introduced in 1989.