Probably degrades with time: Elanra rates their (presumably high-quality) needles at 1200 hours (only 5 months at 8 hours a night). Some companies claim "needles that never wear out", but I doubt that. There's consistent references to "fouling" and corrosion of ionizer needles in industrial applications (for example, see Generating Air Ionization With No Contaminating Particles).
Probably degrades faster when more ozone is being produced.
Stainless steel probably helps the corrosion problem.
Glass plates may produce nitrogen oxides?
Salt lamps are supposed to generate negative ions (presumably much less than a typical ionizer?).
Lenard Effect from splashing water generate them. Speculation that foggers/atomizers/misters might do the same.
How bad is ozone exposure?
"Indoor ozone levels usually measure between 30% and 70% of outdoor concentrations." Does that suggest mild ozone exposure is ok?
Elanra and Friends of Water are spoken highly of by Peat-people.
Individual Peat-fans have used LightAir and Comtech without complaint.
Products that look promising to me
Making an ionizer from parts is appealing to me, both for cost-savings, and for the freedom to replace the needles frequently. It's achievable for less than $10, using parts such as the Seawise Industrial SW750 (available here with some assembly details here. Thanks to Stefan Curl for the idea of hand-making an ionizer, and for the SW750 suggestion.
They claim "ozone emission amount = 0", which is probably untrue. Admittedly, I couldn't smell any ozone even holding it directly up to my nose. However, after opening the ionizer to see the emitter (and possibly damaging it slightly) I think I could smell some ozone. I have a picture of the internals for the curious; it appears to use a bundle of tiny wire emitters. I don't have any evidence for this, but I suspect wire emitters are inferior to needle emitters when it comes to ozone.
I can't see any glowing from the emitter tip, which is probably a good sign that it's not emitting a ton of ozone. Or at least not yet.
The manufacturer suggest that it may last "up to 5 years".
As far as I can tell, Purely Products only makes one outlet-based ionizer, despite some sites showing different names or packaging. The link above is to Amazon, which is the cheapest place I've found it. Let me know in the comments below if you find a cheaper supplier.
Wein's VI-2500 Ionizer is about $117 and claims "no detectable ozone" (I'm not sure the limits of their detection, though it seems like a good sign that they don't make this claim for the lower-end SaniMate model…).
Comtech's IG-133A is about $125, and claims "less than .02 parts per million ozone close to the emitter." Uses wire emitters (presumably inferior to needles?) but are replaceable (!):
CFE (conductive filament) emitter has tiny fibers (0.8 microns), so "requires no trimming". $30.
SSE (stainless steel) emitters do require trimming. $10.
This the brand I'm currently using, see my quick video review here. I've continued to use it in the years since that video, and recently decided to test its ozone output with these test strips. I used them in scenarios outside their specs because I didn't get any detection results with the 10-minute exposure time they're meant for. I did things like enclosing them in a small garbage can with the ionizer, leaving them exposed for 16 hours, etc. As a control, the general indoor/outdoor air in my area appears to have almost 0 ozone. Overall I'd take a wild guess that being an inch from the ionizer in a normal room (with a door open but no outside airflow) has an exposure under 20ug/m3. Which is way below the danger zone mentioned with the test kit, at least.
Lightair's IonFlow 50 is about $280. They do claim "Ozone level: 0 ppm, not even measurable (instrument measures down to 0,002 ppm)", which is refreshingly open compared to other manufactures that just claim "none detected".
Elanra's MKIII seems like an ideal ionizer, but its price tag of $725 seems ridiculous.
Needlepoint, with replaceable needles (!).
Claims "Less than 0.005 parts per million."
They recommend it 1-2 meters away at head height.
Maybe the "personal pendant" would suffice? It's sold out right now, however.