If you follow any Japanese speakers on Twitter, you'll almost certainly see them use なう at the end of a sentence, to say "I am in this place/doing this thing now". Where does this use come from? Who started it? Is it anything to do with the (e.g. bus) announcements that say "なう"?
It's not clear exactly who or what started it on twitter, but なう does indeed come from the English "now". It became popular in 2009, shortly after the release of twitter (according to this site). Here are some Japanese articles exploring the usage:
The なう that you hear in the Bus announcements is actually "なお" meaning "furthermore" or "in addition". If I recall correctly, it is often used in the part of the announcement that is describing the locations near the next stop.
Twitter came from the US, so I'd argue that original Japanese twitter-ers picked it up from the English feeds that they followed. Additionally, "now" is common enough of a word that most Japanese know it in English, even if they don't speak English fully, so I reckon it just caught on like that.
I don't know for certain, but 「ナウい」(and later「ナウな」) was a trendy slang word beginning in the 70's or so. (It is no longer trendy, and is in fact now very dated, so don't try using it to sound cool. :) So there was already a precedence for this word.