What's that weird flare in my low-light shots?

OK, so the title of this post wasn't actually asked to me in that form. I have, however, run across people asking about it, and not all that long ago, I was trying to figure it out myself. Well, with thanks to my friend (and fellow photographer) John Cornicello for originally pointing me in the right direction, I've figured out that it's filter flare.

The article linked above does a pretty decent job of explaining it, so I won't go into more detail here at the moment. If you have any questions about it, though, do of course feel free to ask!