What's better than wifi to connect servers wirelessly (ethernet-like latency)?

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

Maybe you can make use of Power line communication, if the problem is only that you can't install network cables.

There are a lot of wireless technologies but very few, if any, of them will be suitable for your scenario. Cabled connections, whether they are based on copper or optical fiber, provide higher bandwidth, simpler connectivity and far better security.

2.4Ghz and 5Ghz WiFi are designed for general purpose coverage. Typical latencies over a nice clean open air link will be in the sub millisecond range but degrade fairly rapidly, along with effective bandwidth, up into the 10's of milliseconds as you insert walls and other obstacles and force the RF layer to start using its error correcting capabilities. Compare that to wired links where latencies can happily drop down below the microsecond level over short links.

With RF in the 2.4Ghz range you will more easily traverse floors\walls but the 2.4Ghz band is noisy as all hell because it is a traditional dumping ground for "waste" RF - everything from Flourescent lighting, Microwave ovens, Medical imaging gear and Ion implanters were all allowed to use that band provided the power output wasn't too high. In any case they all add to the background level of noise that every other WiFi device that happens to be in the area also contributes to. You have almost no control over that although I suppose you could build a faraday cage around your building if you were determined enough.

The 5Ghz range is much cleaner as it was originally for licensed RF only. The big drawback with 5Ghz in your case is that its range and ability to penetrate floors\walls is a lot poorer than 2.4Ghz. This limitation improves performance in office type areas but is a major drawback if you are looking to send signals through a couple of floors of a multistorey building.

Apart from WiFi there are other general purpose options - HiperLan\HiperLan2 were a European precursor to 802.11 that used\uses the 5Ghz range for example - but WiFi and WiMax have pretty much taken over that end of the market.

Targeted point-to-point Microwave connections have been in use for decades and are still an option but I'd be surprised if they were suitable for in-building use anywhere. Motorola make a bunch of devices that deliver ethernet bridging over various frequencies, there's a lot of detail about their products on this page. These offer speeds up into the 100's of megabit range but the major drawback (apart from cost) is that these all need to be licensed which may be a problem depending on where you are. Latency is stated as <1ms and my experience with this type of link has generally been pretty good but I've only ever used them as part of a metro-area scale link. They're not generally a cheap option either.

If you really do need to support servers on the end of these links I think you should try to get fixed cabling in to provide the connectivity you need - it will be cheaper even if you have to drill holes through the building to make it happen and it will be a lot easier to maintain in the long term.

Is it a point to point link or a star topology you're after, and how many endpoints would you like to connect?

Power over ethernet (powerline) is at least a few ms of latency; do keep in mind it will not work across a lot of power equipment (conditioning, ups's, different phases, fuses, ...).

Another option could be to abuse existing wiring, either for ethernet or vdsl. The latter is arround 6-8ms, modems can be had for arround USD150 a piece. I've seen 100Mbit ethernet runs over very (very) dodgy wiring.

Lasers?

Wifi is really the only way I'd know of. Depending on what you're using it for, I wouldn't use wireless to connect servers. I'd have them wired to the infrastructure, as wireless is more limited for servers for bandwidth under load, easier to interfere with, etc.

If you have a specific set of limitations for your situation, you might want to describe the situation for alternatives instead of just asking for something other than wifi. Sometimes the group here is creative with workarounds for a situation :-)

If, for some reason, the servers can't be connected via copper or fiber, then for short distances I would think wifi would be the obvious choice, but it's subject to interference and eavesdropping. For longer distances when line-of-site access is available, microwave or laser bridging could be used. These are also subject to interference, including by heavy fog. Eavesdropping might be possible, less so with lasers, but would probably be easy to detect if the signal is interrupted due to the fact that they use line-of-sight.

One could even bounce traffic off a satellite if, for example, there's a mountain in the way and latency isn't a problem.

This isn't what you want to hear, but wireless is not suitable for anything other than connection of a few clients here and there. If you need to connect servers and provide a reliable infrastructure you need to be looking at wires.

You may be able to get a powerline adapter system set up as suggested in another answer, if you really can't run wires, but my take on this is that you can either run wires now (whether copper or fibre) or you can run them later after wasting time, money and the confidence of your users on a poorly thought out wireless link.

Anything other than Ethernet or Fiber will be too slow/insecure/vulnerable/weak for any serious server environment. If it is at all possible, your best option by far is to use copper or fiber. There are always creative ways to pull wire in already-finished buildings, but in sturdy concrete-and-steel buildings you will be hard-pressed to get any signal through a floor. I've had trouble with 2 load-bearing walls in such a building.