"Saint James Infirmary" is an American folk song with English origins. In Britain it was called "The Unfortunate Rake" and it was about a sailor who dies after spending his money on prostitutes. In America it became a more drinking or gambling-related song.
The most famous and influential version was Louis Armstrong Dixieland jazz rendition in 1928, but I really love what Jimmie Rodgers did with that song, keeping the melody and the setting but telling a slightly different story. His "Gambling Barroom blues" is the first country version of SJI.
For more details, check out the Wikipedia article, it's fine.
And go to the excellent blog Honey Where You Been So Long to see a list of St James Infirmary versions.
Jimmie Rodgers - Gambling Barroom Blues (St James Infirmary) (buy) (1932)
Please feel free to mention your favorite versions in the comments section