Which openings have a vastly different popularity on the net compared to OTB? For example in about 100 games as white I have never had someone play the Grunfeld against me. Also I had never seen the Svesh in play before using this site.

what world are you talking about?!!

I think I have seen much more gambits of sound and unsound nature on chess.com rather than in tournament play. the only gambits I have witnessed in OTB are the King's and Queen's gambits. not a single Danish! how weird...

At my club I see the French Defense played against me (with terrible success :P) on a regular basis.  Online I rarely see any mainline french lines.

At the club I also see the Grunfeld a lot, I rarely see it online.

Oh and the scotch is WAY more popular at the club than online. 

Edit - TOTALLY forgot about that damned pirc!  I face it all the time in OTB, never online, and I hate it!  I score super well against it but it bothers me to no end that people continue to play it!

My theory: Simpler lines that require minimal memorization tend to be the favourite of the older folk that can be found at a chess club.  Online, people are more comfortable having a battle of opening theory because databases are a click away.

oh! the four knights and the scotch gambit always pop up in the clubs and tournaments. while online the four knights defense is a big "no!" and the scotch is rarely seen.

online I see many players adopt a Pirc... and then train it to be a Lion or a Hippo, thankfully at the local club not many people like to risk themselves seen playing a weird and possibly dumb opening.