"Useful" is quite subjective, but the popular packages are probably useful to a lot of people.
The most popular elisp projects on github can be found here https://github.com/search?l=emacs-lisp&o=desc&q=stars%3A%3E1&s=stars&type=Repositories. But there are three entries right in the top ten which don't belong there.
And then there is http://melpa.org plus clicking twice on "DLs"http://www.melpa.org/#/?sort=downloads&asc=false. Here packages that are still under development tend to accumulate more downloads.
Despite these deficits these two resources should give you fairly accurate representation.