This is OpenWRT, not the same thing as DD-WRT. It's basically a full embedded Linux distro with extra emphasis on networking and complete with its own package manager, so the answer is yes you can do quite a lot with it.

It's possible to do more general purpose stuff with DD-WRT as well, but it's not quite as flexible . IIRC you can install your own packages there too, but it is set up to be more streamlined as a SOHO router.