Update (Jan 2019): Bower hasn't been maintained for years, and using Bower packages should now be considered a discouraged practice.
Napa is a package that lets you install arbitrary projects (without package.json) into
node_modules. Bower packages are repos without package.json manifests. Hence, you can use Napa to use Bower packages as if they were npm packages... sort of.
Promising idea, but I'd like to see it developed more. Some caveats as of 1.2.0: