I’m not even going to attempt to give my own review of this tool!
What I am going to suggest is that you read Alan Levine’s excellent evaluation of VoiceThread at his post Easy Peasy Rich Media – Voice Thread. Thanks Alan for the ‘heads up’ on this at Twitter 🙂
From its site:
A VoiceThread is an online media album that allows people to make comments, either audio or text, and share them with anyone they wish. A VoiceThread allows an entire group’s story to be told and collected in one place.
judyoconnell.com 
agree that Voice Thread is a great web application.
I agree that Voice Thread is a great web application. I’d also like to suggest another one that I think is ideal for English Language Learners called Mixer Cast (http://www.mixercast.com/).
I’ve written a post explaining why I like it so much:
http://larryferlazzo.edublogs.org/2007/05/22/make-an-slideshow-online/