Microsoft has their own search advertising based on the pay per click model.   They call theirs Adcenter and it works basically the same way as Google and Yahoo. 

Although it does not get nearly the traffic of Google or of Yahoo it can still deliver qualified traffic.  One of the reasons MSN Adcenter is remaining smaller than the other two is that it has no partner search engines like Yahoo and Google do.

The interface is friendly and the clicks tend to be cheaper than the cost of clicks with Google or Yahoo.