Official-Typing-Test.com was created out of frustration with the other typing tests on the internet. I was doing tests at about 5 different sites, and my speed was varying 15 WPM between them.
There were some sites where I had memorized all of their tests. Some are just a random list of short, easy to type words. Others will alert you if you have made an error (One popular site even has you type line by line with the test paragraph, alerting you of each letter you have made an error on!). And there are also tests that will not allow you to fix your mistakes at all.
I was getting pretty good at some of the tests, but I found out that I was not typing any faster in real life, so I was wasting my time. I needed something more realistic. In my opinion, if you want a test to alert you when you make an error, that is personal preference. But disabling certain keys, and never having to look away from the text you are reading to correct a mistake are bad habits to get used to.
For the tests, I have found some uncopyrighted old books, where you have to pay attention to the spelling and the word usage, because it is a little unnatural (at least for me). And the test is designed so that the words you type are not going to perfectly line up with the words that you are reading. It will not alert you of your errors until you finish the test, and you are not going to have a second-by-second WPM score on the side of your screen. There are over 20 different tests, chosen at random, and I will add more if I start getting feedback that you have these memorized.
Try a speed test now.