An ordinary man (pet form of the given name Joseph); the usage is recorded from the mid 19th century.
Joe Bloggs in British usage, a name for a hypothetical average man. The equivalent in North America is Joe Blow.
Joe Public a name for a hypothetical representative member of the general public, or the general public personified.
Joe Sixpack a name, chiefly in the US, for a hypothetical ordinary working man; Sixpack refers to a pack of six cans of beer held together with a plastic fastener.
See also Uncle Joe at uncle.