Real typing tests are a lot longer than this little JavaScript gem, but it does the trick, provided you obey the instructions and hit Tab or click "Stop" as soon as you've finished the text. You can even embed it on your own page with these instructions, so here's mine:

typing test (c) CalculatorCat.com

The last time I took a typing test, it was for a government agency. The examiner asked me up-front how fast I typed, and I said, "Probably around 100 words per minute." He looked at me as if I'd told him that his grandmother sold herself to concentration camp guards for pocket change. He officiously informed me that such a speed was not possible. His test would go for three minutes, and count the number of five-character words (including spaces) typed. He'd then divide that by three (for three minutes) and subtract one for each error, giving his version of words per minute.

In the three minutes, I typed 315 words with 5 mistakes, for a speed (using his rating) of exactly 100 words per minute.

He made me take the test again while he watched. That time, I got 101 WPM.

He looked at me with some appreciation, saying something like "Damn, you type fast," and then marked my speed on his official government record as 85 WPM - because 100 WPM simply was not possible, and he damn sure wasn't going to be proven wrong.

(For the record, my speed on the above test hovers between 104 WPM and 114 WPM, provided I stop it correctly.)