The Peroxide Fix
How it works: The PEX-lined steel nipple goes in the hot port. The peroxide is poured in the pipe nipple on top of the valve. The hot water can flow out the side port of the brass tee. One application is usually good for a week or so if you have chronic odor despite use. But when it's stagnation odor, using the water in, say, a vacation cabin, will keep the odor away until you leave again and let it sit idle.
Simple, Cheap, But Not as Easy as an Anode
If you have a vacation cabin that sits idle a lot, eventually you'll have stagnation odor, which smells the same as chronic odor. What's the difference? If you have chronic odor, this will fix it for about a week or so, then it will come back. If you have stagnation odor, this will fix it until you leave the water heater idle for a month or more. So if you can't afford a powered anode for chronic odor, this is a cheaper alternative. And if you have a vacation cabin, it may be the only thing that will solve the problem. If you want to buy this, go here.