More polite: - If you don't mind my saying, ___ . - If I may say so, ___ . More casual: - I know no one asked my opinion, but ___ . - (US English slang) If I can just add my two cents, ___ .

You could also begin with 'I know this is none of my business, but....' This is the usual preamble to an intrusive comment or unsolicited piece of advice. By the way, the British equivalent to 'my two cents' is the rather odd phrase 'my two penn'orth'. It's pronounced 'penuth' and is an archaic contraction of 'two pennies' worth'.