If you want to register new domains in December 2008 from Namecheap, here is the coupon code you should use: THREEKINGS. With this code, you will be able to register domains for as little as $8.81 (instead of the regular 9.69)!
Namecheap occasionally releases renewal coupon codes through their newsletter. You should subscribe to their newsletter from within your account. If your account has more than 50 domains or if you are a Namecheap reseller, you can contact their sales department requesting for a renewal coupon code!