The other suggestions are good. If one is a fan of CLI, then cv provides a few easy options.
Download a published extension from the directory (long name).
cv dl org.example.foobar
Download a published extension from the directory (short name).
cv dl foobar
Download an unpublished extension (long name and zip URL)
cv dl org.example.foobar@http://example.org/files/foobar-1.2.zip
Download a pre-release (alpha/beta) from the directory.
cv dl --dev foobar
Note: By default, all these commands will download to the configured extensions folder, use a sensible subfolder (e.g.
sites/default/files/civicrm/ext/org.example.foobar), and refresh the ext list. You can download to an alternate location (