This is kibitzing. In friendly blitz games it's usually fine in my experience, and can add to the tension and fun of the game. I think it would be pretty out of order if you were watching a Carlsen - Ivanchuk game (for example) and started loudly suggesting moves.
Observation Etiquitte

I don't think the players can see the comments. I've always wondered when people observe my games why they're so silent!
Now I know. They're not!

Oh, you're talking about on Live Chess? Then I think ozzie's right, the players can't see any kibitzing comments.

This is kibitzing. In friendly blitz games it's usually fine in my experience, and can add to the tension and fun of the game. I think it would be pretty out of order if you were watching a Carlsen - Ivanchuk game (for example) and started loudly suggesting moves.
That would be funny though! Actually it would be even funnier if you had Kasparov and Korchnoi doing the kibitzing.
I was watching a high ranked game for the first time today. I was suprised when some other observers started commenting on the action, as to what a given player should or shouldnt do in a given spot.
I have no idea what etiquitte is. Is it OK to comment on an ongoing game?