It seems like there are a lot more Ladybugs (Ladybird Beetles) around this spring than ever before. I hadn't looked into the matter before, and was surprised to learn that several different species of Ladybugs have been introduced to North America from other parts of the world. The reason for their continued introduction is that these species are voracious eaters of other insects, especially aphids. They are often recommended to gardeners as a "natural" way of dealing with aphid infestations.
Unfortunately (and as always), these introductions are having some unintended consequences. For example, the Asian lady beetle (Harmonia axyridis) does not die at the end of the growing season, as most native species do. Instead, they hibernate, and they love finding warm cozy places in which to do this...like in your house! If you are having serious problems with ladybugs, you might want to consider some control methods, but so far they are just considered a nuisance species, and no negative ecological effect has been associated with their introduction.