There are a lot of good answers to this question. However, an answer that actually provides a complete overview is lacking. I recently wrote an article on my website where I covered all of the wild camping rules for every single European country. You can find that article here: Wild camping in Europe: Complete legal overview.
Covering every European country in a single stackexchange answer seems a little bit crazy (I needed 3500 words to cover everything). However I will provide a general summary for every region in Europe in this answer.
Northern Europe is generally very wild camping friendly. There is only one country in Northern Europe where wild camping is not legal, Denmark. But even Denmark provides plenty of alternatives. Certain countries in this area of the world do not only legally allow wild camping, they even encourage it.
Central Europe is not as wild camping friendly as the northern countries, but most countries will at least allow a planned bivouac (one night).
In Eastern Europe, there are not really a lot of laws in place that prohibit wild camping. In these countries they generally don’t have any issue with this. Additionally, there is so much wilderness in these countries that they couldn’t possibly patrol the entire area. Note that this only applies to Eastern Europe, not necessarily to Southeastern Europe.
Southeastern Europe is not as wild camping friendly as Eastern Europe. There are a couple of laws in place that prohibit wild camping. However, in most of these countries, wild camping is tolerated even when it’s legally not allowed. Just make sure to avoid touristic places, beaches and sometimes National Parks.
Wild camping in Western Europe is definitely not that easy. It’s illegal in most countries and only tolerated in a few of them. This isn’t really surprising since this part of the world is very densely populated and there isn’t that much nature left.
Southern Europe is probably the least interesting place in Europe to go for wild camping. In most places it’s illegal, and more often than not, it’s also not tolerated.
Note that finding legal information about every single country is not easy. Not everything in my article might be 100% correct. I might assume that wild camping is legal because I couldn't find any restrictions online + multiple people claimed that it's legal. I'm trying my best to provide a solid and correct overview, but some information might not be completely correct. If you find anything that you don't agree with, please let me know. I mean to optimize this article as good as possible over time.