You can't take a test online. It is illegal. We all know that from the technician questions pool.
Hamtestonline is a method to learn the material for the various tests. It is similar to talking the tests at QRZ but more on steroids. It actually gives you a lot of information on why the particular question is correct. It takes your through the theory and references you to other websites and resources so you can better learn the material the FCC and ARRL wants the student to know. So basically it is an online school for ham radio. I actually used this program to pass my technician in Sept 2006 at Shelby, NC Hamfest, General by March 2007 at Rock Hill, SC, and Extra on Sept 2007, again at Shelby, NC Hamfest. You buy a subscription for $49 that lasts for 2 years. This gives you 2 years to get your licenses and gives you plenty of time to study and practice taking the tests. Plus, it keeps track of questions you miss and provides customized tutorials to direct your learning on those area's that are the weakest.
I recommend
www.hamtestonline.com for learning about ham radio and getting all three licenses.