Media buys allow you to post your banner advertisement on popular websites. In fact I have had my ads run a number of times on MSN's main website. Some media buys cost big but the amount of exposure you get from them often outweighs this cost. Below is a list of a few websites I use myself to buy media on other sites.
BuySellAds - Hosts a wide variety of websites and blogs. I recommend going for ad spaces that are sold weekly and monthly at a time instead of based on CPM. CPM requires a ton of attention to figure out where your profit margins are at.
ProjectWonderful - I love the whole setup of this site and how it all works. Only issue is the site really lacks any major authority sites. However you can find a few decent sites to bid on that are within your niche and lock in huge advertising for a few bucks a day.
Social Networks, Facebook, LinkedIn, IMLoop - Media buys on these sites work out very well, some have become a bit over saturated like Facebook but my LinkedIn and IMLoop ads have produced solid results for their low cost.