MSN
The MSN network offers Pay Per Click advertising on its site, while you may not get as many people using MSN as you do Google.
However this often means that the costs per click are lower, and so MSN is a great place to use Pay Per Click especially if you are on a smaller budget.
The reporting and features are similar to Google and they appear to be working on improvements all the time.
As part of a rounded Search Engine Optimisation and on-line marketing strategy you should use MSN along side the other main PPC providers.