This is a very good resource, but it has limitations from practicing point of view. You cannot use it for (unlimited) practice, like other sites mentioned in this post.

I continuously got "We are in private beta" on their site. I "came across" this site few times and wanted to give it a try. Their CEO (Aliene Lerner) was on few podcasts.

The way I got an account is when I clicked a Google Ad of theirs. Once account is created, I gave a practice interview. Interviewer said I was not ready for real jobs yet. If you do well, they refer you to Google, MSFT and such companies.

The best part (IMO) is your interview is recorded. You can listen and watch your performance anytime later.