To me, a good party/reunion always have one of the famous Johnny Cash songs
(Ring of fire, Folsom Prison Blues, Walk the line etc).
Also, the best thing about Johnny's songs is that they are not the hardest to play. So you can learn 3 or 4 songs pretty quickly (learning the words by heart his the longest thing to do :P )

Like dalduqye said, there's always someone that wants a good Beatles song. I would say that this would be the "slow" song. You always have one of those in a jam session. That way you reach out to almost the entire public/group. Rocking is good but theres always place for a "sitting and listening" song.

Maybe an Elvis song is also a good one to know. That way you reach out to the older generations in the groups. Go check out the lesson on Jailhouse Rock by Anders here on GuitarTricks ;)

Like Victory said, House of the rising sun is a good one that everyone knows.

Remember that your song picks also depends on the region where you live and the people present. For example, if you live in America down south, a good old blues or country song is a must have in your repertoire. Your from Alaska but HEY ! theres always room for blues :)