I saw a question in superuser.com, and the title ran:
What’s the fastest desktop search tools you’ve used on Windows?
tools, is this correct? Or should it be
What's the best tool or
what're the best tools?
What are should be used if it's formal. Informal occasions would accept what're.
Note that in AmEng at least, is is becoming the dominant inflection of be when used with existential there (i.e. There's many ways to do that), so I wouldn't be surprised to see this trend shift to any cases where the correct agreement must be found later in the sentence. AmEng is shifting to a preference for sentence-initial agreement rather than syntactic-role agreement.
You're absolutely right. This sentence has a mistake.
This is called "subject-verb agreement":
Thus, the example OP gave couldn't have been "What's the fastest... tools", because "What's" is actually "What is", and thus, the verb is singular, whereas, "tools", the subject" is a plural.
The sentence as it stands is grammatical. It is written in an informal register, but grammatical.
In a formal register, "What are..." would be used. But there's little reason to suppose that a question on an informal Internet forum "should" be written in a formal register: as far as I can see, the question as it stands is perfectly appropriate for its context.
I don't think people would commonly write "What're..." with a contraction, even though arguably that is a closer representation of the pronunciation than "What are" (similarly, people don't tend to write "Would've", but that's usually how it's pronounced).