ATL sung to the tune of Over There from the WWI era
Stolen from the BigSoccer thread on the same subject:
Send the word, send the word A-T-L!
That the (teamname/nickname) are coming,
The (teamname/nickname/) are coming,
The drums rum-tumming
So prepare, say a pray'r,
Send the word, send the word to beware.
We'll be over, we're coming over,
And we won't come back till it's over
Could probably be refined some more to personalize it to Atlanta and our team even more, but the basic elements work very well and it can easily be learned.