I like Wordahead because it is more professional. Brainyflix has videos made by students, which can be a little distracting when the video is way out there.
Having said that, I am about to contradict myself: they do accept student videos. However, because most of the videos are made by the individuals in control of the site (at this point), it seems more professional. So, for those out there who want students to have the opportunity to create the videos (as with brainyflix.com) but also want the more serious vibe, Wordahead is the place for you.
HERE ARE SOME FEATURES I HOPE THEY ADD:
The ability to make lists of words you might need for a vocabulary unit
The ability to post these lists at another site (which they told me they are working on)
The ability to download them to smart phones, iphones and/or ipods
The ability to create accounts from which teachers can invite students to join their study page
Words that cater to the younger grades (I know it is for SAT prep, but a waterfall starts with the first drop)
As I have already stated, this site is very promising. The fact that they are in the beta stages of testing means that there will be a lot more cool features in the future. I would encourage you (and your students) to check it out.
Here is their widget: (If you want the code for your website, click here.)