popular R packages

When the question arises "How many R-users there are?", the consensus 
seems to be that there is no valid method to address the question. The 
thread "R-business case" from 2004 can be found here:
https://stat.ethz.ch/pipermail/r-help/2004-March/047606.html

I did not see any material revision to that conclusion during the 
recent discussion of the New York Times article on the r-challenge to 
SAS.

Gmane tracks the number of r-help activity (I realize not what you 
asked for):
http://www.gmane.org/info.php?group=gmane.comp.lang.r.general

The distribution of r-packages is, well ... distributed:
http://cran.r-project.org/mirrors.html

At least one of the participants in the 2004 thread suggested that it 
would be a "good thing" to track the numbers of downloads by package. 
I have not heard of any such system being installed in the mirror 
software and I see nothing that suggests data gathering in the CRAN 
Mirror How-to:
http://cran.r-project.org/mirror-howto.html

On the other hand I am not part of R-core, so you must await more 
authoritative opinion since a 5 year-old thread and amateur 
speculation is not much of a leg to stand on.

There are lexicographic packages for R. One approach to a de novo 
analysis would be to do some sort of natural language analysis of the 
r-help archives counting up either package names with non-English 
names or close proximity of the words "library" or "package" to 
package names that overlap the 30,000 common English words. That would 
have the danger of inflating counts of the packages with the least 
adequate documentation or a paucity of good worked examples, but there 
are many readers of this list who suspect that new users don't look at 
the documentation, so who knows?