Ortleib and Arkel are really nice, so is Jandd, but I haven't found any reasonably sized h'bar bag (400+ cu.in. that doesn't tip forward or bounce around on its mounting once you load it up.

My solution was to get a canti-mounted platform rack and MacGuyvered a way to attach the mounting bracket to the rear of the rack. I've got a super cheapo bag (Nashbar Elite, $24 on sale) that sits on the platform solid as a rock. If you don't have canti-brakes, similar racks exists that you can use P-clips to attach to the forks.
Depending on how much you want to spend, and how much jury-rigging you want to do, you can go my route ($24 bag, $7 rack, $5 of stuff from Home Depot) or you can go super-fancy and get a V-O rack with built in decaleur (fancy French name for "QR mounting bracket") and a Gilles-Berthoud handlebar bag and spend almost $400 for the setup.