I stumbled over this great presentation on slideshare offering a great, structured and commented toolbox of free digital research. You need to tweet or share the link to get the full presentation, but it is worth it.
They use Holiday in as a case client for the presentation, so you can actually see real examples of the tools in use. There are listed pro’s and con’s for each tool and it is very easy to understand – even is you are not fully training in digital research.
Some of them are based on US data but mostly they are usable in Europe too.