Myriad web, pro, pretty much the same thing, except web is optimised as a display font for smaller text on screens, and pro is the name for the whole family package with bold, italic, etc.
Everyone should have myriad pro (I think built in with illustrator?) and failing that, a full set of the near-identical frutiger.. I think it's a good font to go to when you want something a little fresher than arial or helvetica.