Well,
download.com has some download stats for it.
But you should try to find other sources to make it more objective.
Also, you should give attention for new software.
MSN Messenger 7.5 is in the list but it doesn't have big downloads because it's new.
So, you need to take account of the release date.
One way I can think of for now is to determine the popularity by total downloads/day.
On the sidenote, the most popular software is not always the best software, imo (i.e. Kazaa).
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