If you're looking for just the right package to solve your R problem, you could always browse through the list of available packages on CRAN. But with almost 6000 entries, that's not going to be the most efficient process. And even then, many very useful packages aren't found on CRAN: there are more than 800 packages hosted on BioConductor and more than 200 commonly-used packages on GitHub (with more than 3 GitHub stars).
Rdocumentation.org, a handy website from the team at DataCamp, makes it much easier to fund just the function you're looking for, whether it's found in a package in CRAN, BioConductor or GitHub. Just search for a keyword, and it returns all of the matching packages, plus the full help packages for each of the matching functions. Or, you can just browse the most popular packages, based on their download statistics from the RStudio CRAN mirror.
(Also check out this poster for an analysis of the top-ranked packages from July 2014.)
GitHub and bioconductor: New! Search GitHub and BioConductor packages on RDocumentation