I have been using this reference frequently, so I thought it might be worth sharing.
It's an online catalogue.
http://www.freestampcatalogue.com/appIt does have Michel and Yvert numbers for most stamps and I have noticed that there are a few Scott numbers listed as well.
The stamps are listed by country, as well as topical.
I hope you enjoy it, and that it will be of use to some of you.