Meaning:

• さ conveys the speaker's mild assertion: "you see?"
明日はさ、土曜日だからさ、ゆっくりしようよ。
Because tomorrow is a Saturday, let's take it easy.
 
• なにさ means "what the heck!"
総理大臣だからって、 なにさ。What's so great about being a prime minister?
 

Gender:  Both men and women use this particle as "you see?"  But only adult women use it as "What the heck!"

Level:  Very informal

Alternatives:

• ね has a similar meaning, although not exactly the same.  Go to ne (ね)

• A male speaker may use よ as an alternative. But よ instead of さ makes the sentence extremely informal.  Only high school boys may use it.  Go to yo (よ)
明日はよ、土曜日だからよ、ゆっくりしようよ。
Because tomorrow is a Saturday, let's take it easy.

Warning: 

※さ should be avoided when speaking to a superior.