We can't speak for the handful of other sites that offer ITIL practice exams, but we know that our ITIL content is reliable- all of the ITIL practice questions were written and verified by ITIL Prime authors who hold an ITIL certification. Another reason, is that we can actually point to the reference material where we based ITIL practice questions on. We're starting to see sites pop up which "recycle" questions from one another; this is a bit suspicous, as a lot of the free ITIL question "sources" are not so reliable.
Over at ITIL Prime, we often look at the multitude of "free" test prep content offered on scrupulous sites with a weary eye. Brain dump sites are popping up left and right, and ITIL test takers are starting to take them seriously. Why be careful around these types of ITIL prep sites? Well, as an example, we performed a little experiment where we downloaded a collection of these low cost ITIL practice questions (we won't say from which site), and discovered that almost 1/4th of the answers were blatantly incorrect. Not something the average ITIL test taker should feel confident in relying on, eh? We saw the same results when buying these question off eBay- a large portion of the "correct" responses were downright wrong. Do yourself a favor, pay the modest amount and prepare for your ITIL Foundation exam with the knowledge that you're getting the right material.