deadsea - 1:53 pm on Mar 9, 2011 (gmt 0)

No extension at all is probably best for usability. Its the easiest to remember and type in.

I have seen some evidence that having .html on your urls increases click through rate from the SERPs a small amount. That was measured before Google started showing bread crumbs in place of the URL in SERPs, so even that probably doesn't apply anymore.