The common origin story for Uncle Sam is that he's based on Samuel Wilson, a meat inspector during the War of 1812. He used the initials "U.S." on the meat barrels, and one of the troops who received the meat joked that it stood for Wilson's own name, hence "Uncle Sam." By the end of the Civil War, he had become the most prominent personification of the nation. His popular image was, not surprisingly, largely influenced by Thomas Nast, the same guy who brought us our modern depictions of Santa Claus and Jack Frost, not to mention the animal symbols of our two main political parties. But Sam has not been the only human symbol of the States. There was also Columbia, a female figure who dates back to the beginnings of the country, and can still be seen in the logo for Columbia Pictures. Another was Brother Jonathan, whose height of popularity occurred before the Uncle Sam character was created, but who stuck around in some form until the Civil War.
Many other countries have their own personifications. England has John Bull, who was used for the same purpose, but still looks less like he wants you to join the military than that he wants you to have some tea and crumpets with him. A popular French personification is a woman who originated during the French Revolution, often dressed in tricolor and Phrygian cap as in XTC's "Then She Appeared." Interestingly enough, she shares the name of Andy Partridge's then wife, Marianne. Apparently Marianne is one of the trampier national symbols, as she is occasionally depicted exposing one or both breasts.
In a link to classical mythology, many of the female personifications are actually versions of Athena, as can be seen in this picture depicting the Triple Entente. From left to right, these patriotic ladies are Marianne, Mother Russia, and Britannia.
If you're at all interested in this topic, I'd recommend this Wikipedia page as a good starting point. Johnny Canuck, Italia Turrita, the Irish Kathleen Ni Houlihan, the Indian Bharat Mata, and the Icelandic Lady of the Mountain are all there.