Another chapter in the joys of home ownership

I just went down to the basement, and my feet encountered damp carpet at the bottom of the stairs. (Why I thought carpeting for the basement was a good idea entirely escapes me now.) And there's still a large pool of water around the drain that normally deals with the air conditioner condensation, so presumably the drain is blocked somehow unless there's something outside that's preventing it from draining.

Unlike the previous times this has happened, it doesn't appear to be a sewer problem. The water appears to be clean and I can't smell anything but general basement smell. Also, there hasn't been any sewage backup into the shower, which happened the two other times. And finally, I'm not overdue in having Ron the Sewer Rat do the annual reaming out.

So -- any idea what else it could be? I've never had any other sort of flooding in my basement.