Melissa Scroggins of the Fresno County Genealogical Society says they are an incredibly useful tool. "You can find your ethnicity breakdown, you can find cousins, you can find skeletons in your closet that you may not have ever known about."
Law enforcement used one of those sites to narrow the search for the suspected Golden State Killer, Joseph James DeAngelo.
"He left his DNA all over the place," Paul Holes, an investigator with the Contra Costa County District Attorney's office explains. "So we had a sample that we were able to generate a DNA profile. We uploaded into an open source database of other similar profiles, and then from there we get a match list, of how much DNA these various individuals share with the (DNA from the) crime scene."
FULL INTERVIEW: Inside the hunt for the 'Golden State Killer' with investigator Paul Holes
Sites like Ancestry and "23 and Me" didn't provide the data to law enforcement.
Instead, investigators used a free, open-source site called GEDmatch that allows users to upload their DNA information, typically obtained from one of those other paid sites.
Genealogy buffs use GEDmatch to help them find relatives of people they're related to, investigators used it to find people DeAngelo was related to.
Among the nearly one million profiles, investigator Paul Holes found connections and narrowed them down by age and location, to DeAngelo.
Officers then kept DeAngelo under surveillance and as Holes explains, they got a DNA match from something he discarded.
"When they discard their DNA in a public place, we can lawfully grab that and test it."
Holes claims it's a perfect match.
Fresno Defense Attorney Ralph Torres it would be difficult to refute that evidence in court.
"DNA evidence, as presented by a prosecutor, is very strong evidence."
The GEDmatch site posted a statement, saying they were not approached by law enforcement, but noted users have been informed their DNA could be used by others. They said those with privacy concerns should remove their DNA profiles from the site.
Melissa Scroggins of the Fresno County Genealogical Society believes some who use online DNA genealogical sites will be concerned.
"There are two groups: one is very excited and happy to have their information out there to find their family and their cousins, and there is a smaller group concerned about privacy issues. Now that it's such a national high profile case I think it will give some people pause."