The big two; Overture and Google AdWords. Then we have many of the smaller PPC companies, many that offer a good ROI. Microsoft has been following the leaders for a while now, ultimately destroying them. So MSN is entering the organic side of things. What about the paid side of search? Makes logical sense.
According to Andy Beal, that just might be the case, he says MSN Search Staffs-up for PPC Product. There is a thread at SEW forums discussing Andy's post, the thread is named MSN Gearing Up To Launch Paid Search Advertising?.