I just came across with this question and it really attracted my attention. I hope I am not too late to answer it. Either way I am leaving my input so that other readers can benefit from it too in the future.
Answering to it, there are two types of online IQ tests: The online versions of the conventional tests psychologists have always used face-to-face, such as the Raven’s Progressive Matrices, and commercial online IQ tests targeted at the general public, widely and freely available throughout the Internet.
If you need a reliable and professional instrument to use with diagnostic or educational purposes, then the latter is the way to go:
However, note you must be a certified professional in the field of psychology in order to use it (in addition, it is not free).
If the use case of such IQ tests is going to be recreational, for training and practice purposes, or just to obtain a broad estimate of soeone’s IQ, then the sites I have found most reliable in the past are the following:
This might be useful for anyone reading this thread:
Sarah-Jayne Blakemore's lab have an open access online non-verbal reasoning (MARS) task, with validity and reliability data. Whilst the data is not normalised across a large population and so cannot be called an IQ test, perhaps this task could be used as a proxy-IQ measure (and participants could be matched on their non-verbal reasoning scores for example).
With regards to grading problems, traditional Online Tests do not provide fill-in-the-blank items. In lieu of this, this research hopes to demonstrate that if properly designed, an Online Test could achieve the same test effectiveness as manual paper-and-pencil tests. An Online Test system was built to provide both traditional and intelligent Onlin...
As the many-core system-on-chip becomes the predominant architecture in safety-critical applications, online testing will be required to ensure availability and reliability of these systems. To be accepted as a feasible solution, any online testing technique must satisfy time and power constraints while not interfering with concurrently running saf...