Yahoo:

* Social Engine
* Many results have ebay listings (a Yahoo partner), del.icio.us (Owned by Yahoo) and Flickr results (Owned by Yahoo)
* Tries to incorporate Yahoo Answers into many results



MSN:

* Product Engine
* Almost always has results for product based searches not found in the other engines
* Starting to use more Wikipedia results as their algorithm matures



Ask:

* Clustering Engine
* Their results try to give you all the surrounding information around your search term
* Trying to get into the mindshare of America


So, as you can see, each engine does provide a different slice of information depending on your search need. Keep this in mind when you are not only looking for something, but when selecting the keyphrase you want to be ranked for.