Negatively charged ions are everywhere. There is a higher concentration of negative ions by the ocean, during a thunder storm, and in my office thanks to my negative ion generator.
Negative ion generators can be made or purchased for relatively cheap - and put out millions of ions per second.
Generally speaking, negative ions are considered good for you (despite the "negative" name). Negative ions can bind to dust particles and are said to give the fresh, enlivened feeling to clean air.
That said, there are three ways negative ions are created:
1. Mechanical action (water rushing over rocks in a waterfall).
2. Electrical action (lightening during a storm or electrical charge in a negative ion generator).
3. Radioactive decay - that's right, a byproduct of radioactivity is the production of negative ions.
This means that if you have something which advertises "negative ions" as a byproduct - and it isn't moving or plugged into a power source - it is either radioactive, or not really producing negative ions. The small $50 negative ion meters are VERY unreliable and I've seen mine fluctuate wildly sitting still. I've already stated I think it's worth investing in a negative ion generator (or a beach house, if you can). I may rig up a large negative ion generator at some point, and a fountain will also work. But that doesn't mean all negative ion sources are desirable.
In fact, at least one in particular is dangerous and should be avoided. There's others that I can think of, which we'll get into later. This guy does a great job summing up the danger in Negative Ion Bracelets - and talk more about how they're created.