Domainz.io - Premium domains, for not so premium prices
Flippa - Flippa is a platform for trading websites.
Newsy - Monetize your un-used domains and turn them into your own content aggregator with 0 code!
Transferslot - Easily buy and sell side-projects
DomainWatch - Domain Monitoring Made Easy
buysellwebsite - Created by two California attorneys, WebsiteBroker.