Someone posted a message praising Duke's Mayonnaise, which he'd ordered from http://www.boiledpeanuts.com, the Lee Brother's Catalog.
While they do have some interesting stuff in that catalog, and I've added it to my bookmarks, the Duke's Mayonnaise can be ordered for only $3.75 a jar (as opposed to $11.95 for two jars from the Lee Brother's Catalog) from Hometown Favorites
which also has the lowest prices around on such essentials as Fox's U-Bet chocolate syrup (for those New York style chocolate egg creams).
Another great one (though not such a great site) is
My favorite spice shop in the USA. They don't have their whole stock listed online, but if it's middle eastern or Indian and you don't see it listed, call. No doubt they have it.
What are your favorite online sources?