There are well over 3000 packages available from CRAN but navigating them all can be a challenge. Below I have listed the main package sites, but have also included several resources that help you to locate the most appropriate package for the task at hand.
CRAN Task Views: http://cran.r-project.org/web/views
Crantastic: http://crantastic.org A community driven site for R packages where you can search for, review and tag CRAN packages.
Bioconductor: http://www.bioconductor.org/ Open source and development tools for bioinformatics and high-throughput genomic analysis.