If you quickly want to compare some modules for their popularity, I have a hint for you:
0) Install a web browser plugin which can manipulate HTML tables, i.e. TableTools2 for Firefox.
1) Go to http://drupal.org/project/usage/
2) Right click the table header row. In the upcoming context menu select: TableTools > Filter > Search filter.
3) Enter "module1|module2|commonTermInSomeModules|…", and you will get a selective ranking, only showing your modules of interest.
NOTE: In other web browser plugins launching (step 2) and syntax (step 3) may differ, but you get the point.
Maybe http://drupal.org/project/usage/ will add such filters one day as a feature.
An alternative is:
1) Go to http://drupal.org/project/modules
2) Search Modules: "module1 or module2 or commonterm", Sort by: "Most installed".
NOTE: 1) The results are rather too many and thus disappointing as:
a) The terms are searched for not only in the title but also the body part.
b) The terms are not searched as words with whitespace boundaries, but rather as wildcards *term*.
2) It's not presented as tabular data, but rather as aggregated node summaries, and you get it in popularity order, but don't see the number of installations nor the module rank, so for relative proportional comparison purposes this is rather unhandy.
