Aspectus is a simple to use AOP library that allows you to inject cross cutting concerns in an easy manner. It was the culmination of a series of posts made on the subject of AOP:
It uses run time sub classing, thus your original code is never touched and it does not need secondary compilation or access to the compiler. Instead it uses simple IL generation to create a sub class of the original, thus minimal speed reduction. As such it's fast and incredibly simple to use. Developing an aspect for it is also incredibly simple as it uses events to allow your code to hook into the original class. Thus you never have to touch IL yourself. However in cases where you need more control, it allows you to inject interfaces, create your own methods, properties, etc., and even inject your own IL into already existing methods. So you end up with a system that's easy to use but still powerful when you need it to be. You can download the latest version of the code from the CodePlex site found here.