You can also get a Celeron-based (for example with a N100) fanless mini-pc meant for use as DIY routers like these and install something like pfSense on it.
Personally my really old router still does what I need so I'm leaving it be, though I've replaced my media box and my NAS with a similar device running Lubuntu but can't really make it also be the router since it only has 1 ethernet port.