Best of the Web are currently running a promotion throughout October 2008 offering a 20% discount for all new submissions and sponsorship advertising.
To qualify for the discounted rate, just enter the promo code ZOMBIE when completing your order on BOTW.
BOTW are also running a free 60-day advertising offer which you can sign-up for. This post will be continually updated to contain information about the latest information about the Best of the Web promo codes.
There is also a Aviva Directory promo code.