How do you spell wifi / Wi-Fi / WiFi?

According to publications like PC Magazine which would be more likely to use the "correct" spelling, the word is spelled:

Wi-Fi

A wireless local area network (WLAN) technology that conforms to the IEEE 802.11 standard. Wi-Fi is the wireless counterpart to the wired Ethernet network, which is the ubiquitous local area network (LAN) technology used in companies and homes worldwide. A Wi-Fi logo from the Wi-Fi Alliance certifies that network devices comply with the IEEE 802.11 standards.

However, WordNet from Princeton spells it WiFi. Based on a quick Google search, most official sites (such as Starbucks) use either WiFi or Wi-Fi. Since there is so much variation, wifi looks fine as it is, and people understand the forms wifi, WiFi, Wi-Fi and possibly Wifi, you could use any of them and still be fine. The key to your use is that you are aiming towards being understood, not necessarily maintaining the trademark. (If we all maintained trademarks, we wouldn't refer to Blackberries instead of Blackberry mobile devices.)

In the past it was "Wi-Fi", but the current trend is toward "wifi".

It's a little like e-Mail => eMail => email.

December 2015 update: A Google search now results in 41% more occurences of "wi-fi" than "wifi". Apparently the trend has reversed.

The correct spelling is Wi-Fi even if wifi is most used.

Google Trends clearly shows this:

If it's technical document, I would use Wi-Fi. But, if SEO is very important, I would use wifi (+other keywords to create a long tail keyword).

Valeria (anyway, on the Tanaza website we use only Wi-Fi)

FWIW, Merriam Webster came down on "Wi-Fi" -- so if you want to cite a source, there it is.

I'd just like to weigh in here. It would seem that Windows 10 prefers "WiFi" in British English and "Wi-Fi" in American English. I noticed this when I changed my display language. I've probably used both orthographies from time to time, but I think I prefer the hyphenated version.

Wi-Fi is a trademark of the Wi-Fi Alliance. The name is sometimes written as WiFi, Wifi, or wifi, but these are not approved by the Wi-Fi Alliance. -- Source

This is a case similar to facial tissues being generically referred to as "Kleenex". There are many examples of a trademark becoming a generic term, in which case the original capitalization or other unique stylization become lost or varied. Other common examples include Aspirin, Cellophane, Dry ice, Escolator, Kerosene, Lanolin, Laundromat, Linoleum, TelePrompTer, Thermos, Trampoline, and Videotape. -- Source

The official term is "Wi-Fi". However, when English users treat a trademark generically, the "rules" for what is proper become the relative prevalence of the unofficial usage.

According to Google's Developer Documentation Style Guide, it should be spelled Wi-Fi.

We use Wi-Fi as the generic term but are keeping an eye on the prevailing usage as with many tech terms.

An often-overlooked aspect of spelling/hyphenating terms is implied pronunciation. In "Wi-Fi," it's natural to read each "i" as "eye," whereas "wifi" is a bit ambiguous and some might read it as "wiffy."

For modern practical style I almost always go with The Guardian, which has wifi.