My 2005 PT Cruiser has this horrid sulfur smell...?

The catalytic converter is definitely creating the smell. But, it may not be the root cause.

Basically, it "could" be the oxygen sensor causing the problem. But, a malfunctioning sensor should cause the check engine light to come on.

Keep in mind that the converter is covered by the Federal Emissions Warranty, which is 8 years, 80k miles.

The converter can cause this smell from getting too much sulfur in the fuel you're using, or from any additives that may have been added to the fuel. It can also be noticed after long periods of idling, or full-throttle acceleration. You can try changing gas stations to see if the smell goes away, or if you recently changed gas stations, change back or go to another one.

It's also possible that it's caused by the engine running too rich (too much gas), which can actually be caused by a bad oxygen sensor, or any of a bunch of other sensors or etc.

Chances are, however, that the catalyst in the catalytic converter is not working correctly, which is causing the smell.

If the converter starts going bad, it may also cause a decrease in performance. If you have decreased performance, it's more likely to be a converter than anything else, but could still be something else causing the problems.

The converter is an expensive item to replace, they can start around $600 and go up from there (easily going over $1000). There are some aftermarket converters available, but they aren't as good as the factory converters and won't last as long.

Oxygen sensors can run up to a few hundred dollars to replace (yours should be under $100 each, though).

Either way, you'll be better off to have it checked out before you start replacing parts that you're not sure will fix it. Most dealership shops charge around $100 to diagnose a problem, many will not charge that if you have them repair the vehicle. If it's covered under warranty, you won't be charged anything for the diagnosis or repair of the covered parts. If it's under 80k miles, then take it to the dealer to have it diagnosed.

No, it is the catalytic converter. There is nothing else that causes that smell, and O2 sensors don't give off smells as there's nothing inside it to catalyze (hence the name) or to break down, so it cannot give off a sulfur smell.

No,

You could have a exhaust leak & it's venting inside your car.