Adcash supports both advertisers and publishers, offering a variety of formats including banner, interstitial, in-page push and pop-under ads. There is no minimum traffic requirement for publishers.
Adfly is a URL shortening service that pays you for visitors that click on your shortened links. They also sell bulk traffic which you can have targeted to specific regions or countries.
Clickadilla provides advertisers and publishers with a variety of ad formats including in-stream video, web push, popunder, native ads and banners.
Fiverr can be a great place to find traffic providers that are in different niche markets and some that have unique sources of traffic that a larger company may not have the time to focus on, as well as a great place for those selling traffic to make their first sales. They have a review system which helps to weed out bad providers and it is wise to avoid listings with previous mediocre or poor reviews, and there are also a large number of people re-selling traffic bought from a wholesaler on Fiverr, and with that in mind you will want to look for sellers that give a good background to their traffic source and who appear to be the actual owners of the traffic source.
Udimi is the world's largest solo ad marketplace, which offers a place for sellers to post their serviices and for buyers to purchase advertising, which is all handled through a secure escrow through Udimi. Udimi also offers a nice affiliate interface and resources for those wishing to promote it.