This was touched on in a comment on #480, but I thought it's worth pulling into its own separate issue.

Currently, each app that uses Yarn (or npm) has its own node_modules directory with its own copies of all the modules. This results in a lot of duplicate files across the filesystem. If I have 10 sites that use the same version of Jest or React or Lodash or whatever else you want to install from npm, why do I need 10 identical copies of that package's contents on my system?

We should instead consider extracting packages into a central location (eg. ~/.yarn/cache) and hardlinking them. Note that this would be a hardlink rather than symlink, so that deleting the cache directory does not break the packages.