I wouldn't rule out plagues. Yes, no plague has ever been that catastrophic but that doesn't mean it's not possible. While no such thing could occur naturally (there's nothing close in nature) it could still be an accidental release of a bioweapon while it's developers are still working on a vaccine.
Think of a bug that doesn't kill in the usual way but instead causes paranoid schizophrenia and overdrives the heart. You have patients that resist treatment and flee (bringing the bug with them!) but which aren't so ill as to be incapacitated. They eventually die of a vascular rupture somewhere. It would also have to be something that spreads incredibly well.
Even more destructive would be a pathogen that drives it's victims to incredibly destructive acts. (Say, it leaves them believing everyone around is an enemy and that they only have a day or two to live anyway.) If you have a whole bunch of people trying to kill as many as possible you'll see a lot of death and a lot of destruction of technology. The two-man safeties on nukes mean no nuclear war (if everyone's an enemy they certainly won't be able to work with them to launch a nuke) but expect a lot of conventional weapons to be used and an awful lot of non-weapons. (Think of tens of thousands of Bophals. There will be a lot of engineers who understand how to create catastrophe from the systems they normally keep safe.)
As for other approaches there have been multiple mentions of impact events. However, a sufficiently large body can be deadly without it being an impact event. A neutron star or black hole could come traipsing through the solar system with very little warning. Imagine something coming in at a high angle to the ecliptic, for a neutron star the jets are aimed far from the Earth and for a black hole it can't have much of any matter along with it. A sufficiently near miss will do very bad things, the closer the pass the worse the Earth fares, for a very close pass the rubble that's left from passing within the intruder's Roche limit is ejected from the solar system.
A nearby supernova does not meet the requirements--there's nothing close enough that would fry us that badly that fast. Blowing off the ozone layer won't get the requisite kill.