Constant Contact – great email newsletter service (get 60-day free trial)
Google Analytics – amazing website statistics for free
PayPal Website Payments Standard – affordable e-commerce to sell physical products
E-Junkie – affordable service to sell downloadable products
U.S. Postal Service Online Postage – print postage and professional labels on your own
Kayak.com – compare airline travel costs quickly
TitleZ.com – cool tool to search and compare Amazon rankings
BookStatistics.com – facts and stats about books and publishing
Publishers Weekly – tons of publishing industry info