If you use Iron, the version of Chrome that is privacy enhanced by default, you can install plugins from the official Chrome Webstore.
Proxy Switchy for example, uses this URL for browsing from the webstore:
But this is the actual installation URL for the CRX file:
Note the parameter after the id%3D in the url. Once you find the extension you'd like to install in the Webstore, simply replace the red text in the installation URL with the portion of the Webstore URL that looks like random letters and numbers such as the gobbledygook below, and paste the url into the iron or chromium "OneBar", then it should install the extension.
These ID's are thirty-two characters long and made up of lowercase letters.
There's a page on the Gentoo Linux Wiki that explains this as well.
Febuary 4, 2015 Update:
It appears that this method no longer works for some reason.