USB support
Of the Supported Devices, just a few support USB. These include the:
ASUS WL-500g Premium V.1 (German) and ASUS WL-500g Premium V.2
Asus WL-500W
Asus WL-520GU and various devices in this same series
Asus RT-N16
Buffalo WZR-HP-G300NH
Linksys WRTSL54GS
Linksys WRT350N
Linksys WRT600N
Linksys WRT610N Versions 1 and 2
D-Link DIR-330
D-Link DIR-632
Some versions of dd-wrt have USB support built in and can be enabled via the web-GUI, but all versions of dd-wrt can have USB enabled by installing the required kernel modules to JFFS, then using a startup script to mount individual devices. See USB storage for instructions and example scripts.
The cost of USB-capable network routers starts at around $26 for the TP-LINK TL-WR842ND (as of November 25, 2014), depending on capability, but the ability to add external hardware easily makes these far more powerful units.
[edit] USB devices