So (word)

So is an English word that, apart from its other uses, has become increasingly popular in recent years as a coordinating conjunctive opening word in a sentence. This device is particularly used when answering questions although the questioner may also use the device. So may also be used to end sentences. When ending a sentence, it may be:

a coordinating conjunctive to refer backwards to something previously mentioned
a coordinating conjunctive dangling "so" to refer forwards to something that may be said
an intensifying adverb.

Sentence opener

The first known written use of so as a sentence opener is in several lines of Chaucer's Troilus and Criseyde, published in the mid-1380s, for example:
So as a sentence opener has been used in later historical literary works such as:The Rape of Lucrece, 1594, by William ShakespearePamela: or, Virtue Rewarded, 1740, by Samuel Richardson
It is widely believed that the recent ascendancy of so as a sentence opener began in Silicon Valley. Michael Lewis, in his book The New New Thing, published in 1999, noted that "When a computer programmer answers a question, he often begins with the word 'so. Microsoft employees have long argued that the "so" boom began with them.

Purpose

Various suggestions have been made as to its purpose:

as a coordinating conjunctive to refer backwards to something previously mentioned
as a discourse marker
to signal that the following words are chosen for their relevance to the listener
to provide a small amount of extra thinking time

In his Modern English translation of Beowulf, Irish poet Seamus Heaney uses "So" to translate the single-word opening line, Hwæt!. He explains that "in Hiberno-English Scullion-speak 'so' operates as an expression that obliterates all previous discourse and narrative, and at the same time functions as an exclamation calling for immediate attention. So, 'so' it was".

Sentence closer

Referring back

"So" may refer back to something previously mentioned, such as:

"If she notices, she never says so."
Speaker 1: "Has somebody called an ambulance?" Speaker 2: "I believe so."

Other possibilities include:

Dangling so

A dangling "so" in conversation invites the listener to articulate or consider the implications of the information provided without the speaker having to articulate it himself or herself. It has been interpreted as sometimes a form of bragging.
A dangling "so" in conversation may be represented in text as "so" followed by an ellipsis: "...". Examples of dangling "so":

"Yeah, it's pretty exciting, though we're not really sure whether it will work out, so..."
Speaker 1: "How was your date?" Speaker 2: "Well, he didn't show up, so..."

Intensifying adverb

"So" may close a sentence as an intensifying adverb, such as in "I love her so". "So" in the middle of a sentence can also be an intensifying adverb, such as in "I so love her".