"You can turn your blue Linksys router into a Linux box with OpenWrt, an embedded Linux distribution for Linksys WRT54G and WRT54GS routers. This tiny distribution exceeds the default firmware functionality in many useful ways. Instead of having only a Web-controlled wireless access point, OpenWrt provides you with a fully interactive Linux system. Some notable features are the ability to telnet/SSH to your router, install software such as Snort, Mini-Sendmail, and Asterisk, and create and control VLANs for every Ethernet port on the device.
"Though OpenWrt runs on Linksys routers, it can also run on a plethora of similar routers from other vendors..."