Some routers have the capability to enable file sharing.
The Netgear WNDR3700 Router, for example, is a wireless dual-band (offers both 802.11b/g and 802.11n) router with a “ReadyShare” feature for sharing a USB storage device over the network and via FTP. The Linksys Dual-Band WRT600N is a similar router with network storage capabilities. Although using an external hard drive connected to your router will be slower than a dedicated NAS, this option may be less expensive if you already have an external drive to use and/or the router.
Two good routers (compare them here, are) Linksys ac4000 vs nighthawk x6