Online services for multiple consoles are undergoing outages and other difficulties due to holiday traffic, according to their respective service status pages.
According to Nintendo's support page, Wii U services are completely inoperable at the time of this post. Service for the Nintendo DS, 3DS and Wii, meanwhile, are functioning with some success.
Online services for other consoles appear to be experiencing similar stress. According to the Xbox Live Service Status page, Xbox One users may have difficulty accessing game features.
The PlayStation Network is online and operating normally at this time, according to its support page.
We will continue to update this post as problems are resolved or reported.
Update: Xbox Live services are now running as usual. Nintendo's eShop for Nintendo DS, 3DS, Wii and Wii U are still experiencing difficulties.