Within the EU, for EU nationals, there is a right to freedom of movement, which (slightly simplified) means you can visit, live and work in any other EU country. Depending on the country you go to, you can't always just rock up and stay there without doing anything, as there are things like Directive 2004/38/EC which means that in some countries, you may have to register. There's also the whole tax thing that gets involved if you stay too long...
I'm thinking of going to France for a few months, which while not long enough to qualify as an expat under our rules, I don't want to find myself in trouble if the French government has different ideas about where that threshold lies!
As an EU national, if you're going to be in France for a few months, but not working for a French firm, what (if any) are the rules that apply to you around registration / residency / tax / etc?