Da dove viene / chi ha inventato la pronuncia standard?

[Koko]

Oh, right

[Koko]

When speaking Italian do you have to realize the /s/ into [z] when intervocalic? Or may one leave it as /s/?

[Itikar]

It depends from which standard you choose.

In traditional pronunciation there are several rules, not too simple, that help you to recognise when an "s" is voiced or unvoiced. In general, however, for several words you have to remember how to pronounce each word with intervocalic "s", not differently from [e]/[ɛ] or [o]/[ɔ].
Most monolingual dictionaries show you how the "s" had to be pronounced in traditional pronunciation.
As for free online resources for traditional pronunciation there is this site: http://www.dizionario.rai.it/
In general anyway in traditional pronunciation the majority of intervocalic s's tends to be unvoiced.
However even in Tuscany the pronunciaction of intervocalic s's (sadly) tends to become often voiced.
So, unless you are far more interested in Italian literary registers and Italian literature than in contemporary Italian, you may opt for modern standard.
Traditional pronunciation is generally close to the speech of educated Tuscans, and especially Florentines.

As for modern standard, well... it is not a standard yet but only a proposal supported by a famous Italian phonetist (who based this proposal on Italian language used in the media and on Central Italian accents in general). The problem is that in the last years regional pronunciations are becoming more and more accepted on radio and television, so some people argue that this new standard is neither necessary nor desired. But I believe it can still be a good option for a foreign learner.
As for dictionaries that will give you this pronunciation there is not much out there yet, also because the differences between this and traditional are not so big after all.
One of these differences however concerns the pronunciation of intervocalic s's, which in modern standard tend to be voiced (i.e. [z]) although there are several words where they are unvoiced ([s]).
The 2011 edition of the Zingarelli makes a distinction between "s's that are always voiced, s's that are always unvoiced, and s's that are unvoiced in traditional and voiced in modern pronunciations".
While for online resources there is that phonetist's dictionary freely available online: http://www.dipionline.it/dizionario/
(You can find in it also several other variants including traditional and some common colloquial and literary variations)
This "modern standard" pronunciation roughly corresponds to the way educated Romans speak.

So basically the choice is between "lingua toscana" and "lingua toscana in bocca romana".

As for the pronunciation of any s either voiced or unvoiced: both are definitely substandard and unacceptable from a foreign learner. The latter is common in North-Western Italy, especially in Piedmont, while the majority of Northerners pronounce intervocalic s's as in modern standard.
The former is common in the far South instead, and especially in Sicily.
Although consider that even in Piedmont and Sicily this features concern only a part of the speakers; I have personally met Piedmontese and Sicilians who pronounced intervocalic s's as in modern standard.

P.S.
A geminated intervocalic s is always unvoiced, in any Italian variant.

I hope this is helpful.

[Koko]

So, you can pronounce singular intervocalic s unvoiced if you wanted? Sorry, I couldn't follow too much of what was said; not because it was difficult, but because there was too much. From what I understand of it, it truly was a big help ^^. Grazie!

I don't understand why the <s> couldn't have kept it's /s/ only feature :/

[Itikar]

I don't understand why the <s> couldn't have kept it's /s/ only feature :/

Well, me neither...

Anyway, they have never been only /s/, there have always been the two sounds. In modern standard, however, the /z/ has become far more common than the /s/ in that position, whereas in traditional they were kind of fifty/fifty.

But there is nothing that forces you to accept this "new standard" if you like the former more.
Just follow the first link above and pronounce Italian words as in the audio recordings that you find there. Those pronunciations are perfectly acceptable and if you like Italian literature they are probably also more useful to you.

[OldBoring]

Si può provare a dare un'occhiata qui: http://www.achyra.org/cruscate/viewtopic.php?t=1320