FluentIL is a open-source .NET library written by Elemar Junior and Márcio Althmann that makes easier understand, write and mantain IL code.
There are a lot of blog posts explaining FluentIL development process. (portuguese only)
All binaries are avaliable as NuGet packages. You can easily import and use FluentIL on your projects.
FluentIL works fine on basic .NET emitting libraries AND with Mono.Cecil.