I ran into this cool looking spider in my yard in Banjaluka, Bosnia and Herzegovina. It's about the size of a fingernail and unusually shaped (spherical). Could it be poisonous?
The white cross on the body makes me think it could well be an Araneus diadematus. It is quite common and not dangerous for humans. It paralyzes its preys using some venom, though.
I agree with Araneus diadematus. It's a fairly variable spider, but there aren't too many other options that I know of in the area. Caveat - I'm not completely familiar with European spiders, and it's possible that there is another, similar Araneid in the area that I have not come across. It's venomous, but like all known orbweavers, it's quite harmless to humans (and, indeed, to anything too large to get caught in the web). Here's a comparison photo from Bug Guide - the spider in the photo is from the US, but the species is found all across the northern hemisphere. The link goes to some information, and I've included another link to a good website on European spiders.