Shanghainese (上海闲话 / Zanhe-ëwo)

Shanghainese is a dialect of Wu spoken by about 15 million people
in Shanghai. There are also many Shanghainese speakers in Hong Kong.
There is no standard written form of Shanghainese and it rarely
appears in writing. Though the language is the everyday spoken language
of Shanghai, it isn’t used in education and is only occasionally
heard on local radio stations.

The Common Chinese Language movement was started by Shanghai
intellectuals and writers during the early 20th century to create
a common vernacular medium for national communication. Mandarin
was selected as the base, due to the large number of Mandarin
speakers in China and its relative simplicity. The bulk of
vernacular Mandarin Chinese literature were written not by native
Mandarin speakers but by native Wu and Shanghainese speakers.
As result, a lot of today’s Mandarin Chinese vocabulary
comes from Wu Chinese via these literary works. The words and
usages have become so well adapted into Standard Mandarin that
most speakers assume they are indigenous to Mandarin rather than
being cognates of Shanghainese.

What the early Shanghainese proponents for a common Chinese language
did not anticipate was that Standard Mandarin’s promotion would be
handled through the simultaneous oppression of all other Chinese
regionalects, and most harshly on Wu and Shanghainese.

Recently Shanghainese has become popular in Shanghai’s underground
music scene as something that challenges the dominance of Mandarin
and Cantonese in music.

Source: http://www.zanhe.com/general.html