wget is what you want. There are pre-compiled versions for the OS X Terminal. It requires a little finessing to get a functional version of an existing site (and in cases where it's merely retrieving dynamically recreated data might not always be possible), but it's usually pretty doable. Just read the --help options and see what you need. There are a couple presets available, intended to help with common tasks, but you might need to look into creating your own set of customized rules to get the result you want.