There are two things to get out of the way: the first is that there are entirely too many online "IQ Tests" to possibly discuss them all and the second is that even if a website claims that their test is different and just as valid as one that would be administered by a Psychologist, it's very, and I mean VERY difficult to believe that it could be.
Most of what makes an IQ test valid is the normative sample. During development of a test, the publisher administers the test to a large sample of people to get an idea of how we would expect anyone to do (what’s a good score? what’s a bad score?). Your score on a real IQ test reflects a comparison to hundreds or thousands of your peers. They are also selected carefully so that they come from different places, have different levels of educational attainment and otherwise represent the country as a whole. No online IQ test I emailed (and I emailed five) responded with information about how the normative sample for their test was developed. I suppose it is possible that one of these companies went far and wide and invested millions of dollars in developing a sample, but it would make more sense for them to just assign scores based on whatever standard they make up rather than to spend a lot of money on something most people won’t even know they need.
It is also possible that a site could be using people who seek out the tests as a comparison group, but this is also not going to create an adequate sample. If a real test's sample can be accurately described as "a representative sample of the entire country," then the sample for an online "IQ test" will be more like "people who felt like taking an IQ test and have an internet connection and happened to click on that test." Not everyone has an internet connection, and we don't know who would decide to take the test. I would be reasonably sure that it's not students who may be eligible for special education, or the majority of people who are average and have no particular need or desire to know their IQ. Even if an online test had enough people to make a normative sample, then they wouldn’t have the right people to do so. Then again, as I looked at these, I started to realize that there’s no reason for them to do so.
Every online test I took gave me an initial score and then offered me the option of a detailed report for a price. I am inclined to think that the average person would be more willing to spend money at a site that tells them what they want to hear, and who doesn't want to hear that they are a genius? A few dollars for the details of my own genius sounds like a good deal. In other cases there might be motivation for a site to give an artificially low score. Several sites also offered games or other services that claimed to boost your IQ, so why not sneak in a claim that some area could use a little work? This is a big difference from my own practice or the practice of other School Psychologists or Psychologists. We don't make more or less money depending on the score, but we can be sued if our score proves inaccurate. Our biggest incentive is just to provide an accurate score. Setting aside all of the speculation about intentions and how a website might try to make money this way, there is also one inescapable fact: all of the tests I saw were pretty bad.
IQtest.com gives a test of 38 True or False questions from which 13 sub scores are derived. On the WISC-V, (the most common IQ test for children) 5 sub scores come from 10 tests on which an average student will respond to between 4 and thirty questions. These are given in a dynamic manner where the student answers as many or as few questions as they can manage. This means that whereas on a real test, the student goes through a range of questions on each subtest, with at least two subtests combined to make a score in a given area, the IQtest.com test creates sub scores from two or three questions. I could literally click wrong and vastly alter my score. Other sites offer similarly limited numbers of questions, with the most impressive number being Queendom.com's test with 57 questions.
It's possible that there is, or will be a website with the motivation and capability to make a valid IQ test to be taken online, but it does not seem likely. A big enough normative sample to create valid scores would require a huge investment, and accurate scores would overwhelmingly be in the Average range, which is not what people probably want to hear, (right at the top of the page, IQtest.com reminds you that you are "Smarter than you think"). There is also no reason to make a set of tests, with lots of questions that have been heavily researched when a few dozen questions pulled from nowhere look official enough. If anyone cares to take one, I wouldn't stop them (it was fun reading ten different times that I am a genius), but I'd warn them not to take it seriously.
If you have a questions of your own, and especially if you are in Pennsylvania where I am certified to practice, then feel free to get in touch: Greg@DrGregPhD.com