This site was the first suggestion to come up on Google for "online defensive driving" courses. I registered for a $25 class (supposedly the lowest price you are legally obliged to pay for defensive driving in Texas) Based on the portal page, idrivesafely has an A+ rating with the Better Business Bureau and has had 2,000,000 customers since 2003.
I've taken online defensive driving maybe 2 times in my life to get speeding tickets dismissed. I do not remember the previous occasion except for the vague memory that it is (1) time consuming, (2) redundant and insulting to the average person's intelligence, (3) antiquated with 80's footage of crash test dummies and drunk driving videos.
As a side note, I do not understand the point of online defensive driving, because you can keep the window open and multitask on checking your Facebook page, paying your bills online and playing Farmville while the obligatory timer runs-out on each page (by law, there is a minimum time limit each course has to cover). I do understand that the convenience and accessibility of online defensive driving courses makes a bad driver more likely to take one.
The defensive driving course for Texas I took with this site had 8 chapters, each with several subunits. It took approximately 1 hour to finish each chapter. Half the units in each chapter are video based, created with digital animation sequences that feature horrendous crashes that will stick in your memory for a good while, while the other half is based on lengthy, boring, repetitive text. (you can pay $5 to get an audio recording of the text portions). In order to advance to the next chapter you have to score a 70% or higher on 10 questions. Otherwise you are required to repeat it and will be cursing the screen with as much rage as you experienced when you got that damn speeding ticket. Most of the questions are intuitive and pretty easy.
I do have to say that I learned a handful of new things from the 8 hour long course. I prefer a more interactive interface to entertain my attention deficit, but in the end, not bad.