If you don't have tac on your system (which some of our AIX boxes don't), you can also use this short script,

#!/bin/sh

file=$1;
i=1
max=`wc -l $file | cut -d' ' -f1,1 `

while [ $i -le $max ];
do
        tail -$i $file | head -1
        i=$((i+1))
done
[download]

Call it much like tac,

$ ./reverse your.file > reverse.file
[download]

It's not as fast as tac obviously, but if you don't have 'tac' and want to reverse a file it does work.