www.usedrouter.com  Has probably the best deals on Cisco routers other that ebay.  All routers come with a 30 days guarantee.

www.iqsale.com/  Has average prices and includes a warranty.  Sells 2500, 2600, 3600, 4000, 7000 and more. 

www.netfix.com  Has used Cisco routers and switches for average prices.

www.kg2.com/  Sells new and used Cisco equipment.  

www.cisco-routers.com  Site where you can purchase Cisco equipment along with many other vendor's equipment

www.usedcisco2501.com  Sells only 2500 series routers 

www.alliancedatacom.com/new-cisco-list-prices.htm  Many different Cisco products for sell (new and used)

http://www.networkhardware.com/  Sells and Buys used and new Cisco and other vendors equipment

http://www.ciscorouter.com/  Sells Cisco routers, switches, hubs and more.

http://www.alliancedatacom.com/manufacturers/cisco-systems.asp Not the cheapest site but does have a very large selection of routers and modules.

http://www.tcisave.com/cisco_routers.htm  Sells used Cisco products.  Fairly large inventory but prices are a little bit high (but they do come with a 1 year warranty unlike most places)

http://www.itparade.com/  Sells Cisco routers, switches and more.  Doesn't list any prices and inventory is fairly small.

http://www.cccmn.com/  Sells Cisco routers, lists their products but without any prices.

http://www.lordsnet.com/usedcisco7000.htm  Sells Cisco 7000 series routers.