Hi-vis vest - cost almost nothing and can be useful (fog, rain, or regulations that force you to have one).
Spare bulbs - also, cost little, are useful and need to be on the bike in some countries.
First aid kit: I always carry it, though not sure it is obligatory in Europe.
Breathalizer?! To test myself for drinking and driving?!? Did I get this right, it is used for such a thing? Fuck 'em, I'd just avoid France then.
Vignetes are obligatory on motorways only (Chech Republic, Slovenia, Austria, perhaps a few other countries).
On serbian motorways, pay toll is the same for bikes and cars, but if you just go round the ramp (there's alway room for a bike to pass), no one will pay any attention to you. Speed limit is ridicuosly strict, but cops will agree to let you go most times, for 10 - 20 euro bribe. Also, if you're on a motorcycle, just wave and go on when they try to stop you. Capital has a few cops on fast bikes, but few will bother to chase you, or even write your plates down. Just go.
In Hungary cops also accept bribe gladly.
In Croatia, some do, some can make a fuss out of it.
In Serbian parts of Bosnia, they are more than happy to be treated, while in Bosnian (federation) parts, it's a no-no!