The other day I was looking for a Cisco IOS 15 license matrix and was unable to find anything definitive and all that I was doing was trying to find out what services are enabled in different images. Granted, one can use the Feature Navigator to find out and compare, but sometimes we just need something quick and simple.
In IOS 15 there is a Base image, a Security Image, a Data Image, and a Unified Communications / Collaboration Image.  Each one has some common features, but for the most part each are different.  Below is a matrix that I was able to put together using the Cisco Feature Navigator and comparing all the images to each other.  If you would like an Excel version, you can click here

I had to remove the in-line matrix for the moment as it was just too big for WordPress to handle.  For now, just grab the link above.
Also, I think I need to check a few things with some of the other hardware out there.  I think the Cisco 3925e that I selected has much more features in the base code then say, a 2901 would have.  I will do an updated post shortly on some of the model comparisons and IOS feature matrix.