Routers on the Internet

The vast majority of routers on the Internet are Cisco (predominantly GSR 12000 series routers).  They use a CLI (Command Line Interface) that is universally understood by the technical community.  

The second largest presence are Juniper routers (the M160 is the most popular backbone router).

The third, and much smaller portion of Internet Routers are old, legacy routers made by Bay Networks (since bought by Nortel). Bay is cheaper than Cisco; pretty responsive to customers (though Cisco is as well); and almost all configuration is done through a GUI (windowing) interface that drives most routing engineers nuts. Bay claims they're working on a command-line interface, (BCC, or "Blatant Cisco Clone"), but in the mean time most are throwing money at Cisco. It's much easier to debug BGP or other routing problems from a telnet session or over the phone than it is to have to guide someone through a GUI to examine or reconfigure a router.

The third portion of routers are PC’s, UNIX, and LINUX boxes using a T1 IP routing card (manufactured by Riscom, ET, etc.).   They use program called “gated” to manage BGP.  You can build cheap PC routers that route Ethernet and t1 and have more than enough CPU and memory to handle all the routes you'd need for quite some time - but you've then got hardware that's not really as tested or reliable as a Cisco or Bay router.