One way to make use of old routers is to use them as extra wireless access points. You can run the old router out with a cable to a specific area that needs coverage. This can help increase the overall range and/or performance of your network. Plus it can prevent wireless g clients from connecting to the new router, slowing down the network.
Just disable DHCP on the old router and connect the Ethernet cable between the LAN ports of the routers. Consider running the cable in the attic by drilling holes in the ceilings of closets or running it in the basement or crawl space. Then you can connect to the old router and be on the same network.
If you don't want to do a cable run, you may be able to turn your old router into a repeater by replacing its firmware (essentially its brains). See if your router supports the DD-WRT, Tomato, or seavssoft replacement firmwares. If you don't want to bother with third-party firmware, you can buy a repeater or range extender off of the shelf or online. Just keep in mind that users connecting to the repeater would have much lower data rates or network speeds.
Use your power or cables lines to extend coverage
If you don't want to run cables or sacrifice performance by using a repeater, you can still extend your coverage using your existing cable or electrical lines. Powerline adapters, such as those certified by HomePlug, plug right into regular electrical outlets to transmit and receive data via the building's power lines. Similar adapters also exist for your TV cable outlets and are certified by Multimedia over Coax Alliance (MoCA). Though using these two technologies is more expensive than buying a bundle of ethernet cable, they're much easier to install.
You'll need a basic adapter, which runs $40+ for powerline, to pump the network connection from the router into the electrical or cable system. Then you'll need one for each outlet where you want a network connection. Instead of having to use a separate access point or wireless router, you can buy an adapter with one built in for $70+.
Eric Geier is an author of many computing and networking books, including Home Networking All-in-One Desk Reference For Dummies (Wiley 2008) and 100 Things You Need to Know about Microsoft Windows Vista (Que 2007).