Triangulate cobweb spider Steatoda triangulosa
Someone needs to do something about spider common names. I complained about the name inconspicuous running crab spider being too multisyllabic, and "triangulate cobweb spider" is not much better. The other variants include "triangulate house spider" and "triangulate bud spider" (no explanation about the "bud" part, widely copied and pasted from wikipedia). Most people will be content to simply call this a house spider or a cobweb spider, and they would be just as correct if not as precise. These spiders live in houses, and they spin cobwebs.
The triangulate cobweb spider has more or less triangular markings on its abdomen (there seems to be some variability--my specimen's triangles are almost crosses) which are observable only when the observer gets closer than most people like to get to spiders. This very common and widespread spider is completely harmless to humans, and will prey on other spiders, including those very few species which are dangerous. It may be native to Eurasia, but it is now found more or less everywhere there are buildings. Its close relative, the northern false widow, is also fond of living indoors.