Komarketing have compiled a list of all the top resources available on proving/exploring the ROI of blogging. Particularly useful if you have an obstinate C-level in need of justification. There are 27 resources in total, so you can really go to town building up a case.
One thing I’d say – there is always the vague and woolly indirect benefits of brand engagement. For the vast majority of enterprises, this is practically impossible to measure.