I fuelled the latter bits of a 200km at the weekend with Strawberry Sensation Pop Tarts. 400 calories per 2 tart foil pack, which conveniently slips into either the jersey back pocket or the side pouch of my Carradice. Not to be knocked and cheaper than SIS energy bars (basically 50p per tart).
Those are the bog standard UK ones though (chocolate and apple are also relatively easy to find). Our American cousins have a much wider range to choose from. The best I've tried to date are Chocolate Smores flavour, which have chocolate and marshmallow betwixt Graham cracker pastry goodness. I've yet to brave cookie dough flavour or peanut butter, which I suspect will be off the scale in energy density.
Sing after me, to the tune of Star Spangled Banner:
"Nom nom nom nom nom nommmm, nom nom nom nom nom nommmm, nom nom nom, nom nom nom, nom nom nom nom nom nom nom."
http://www.listchallenges.com/pop-tarts-flavors