The Kobayashi Lab [JP] at Tokyo-based Waseda University has been working for years on what are probably some of the most advanced conversational robots out there. The first of this kind of robots, named ROBITA, was first shown to the public in 1999 (pictured above) and was relatively simple by today’s standards.
But ROBITA was already able to detect faces of human conversation partners, differentiate between a number of voices, “understand” simple sentences spoken by humans and react verbally (unfortunately there’s no video available).
In the video embedded below (subbed in English), you can see how ROBISUKE, a further advanced robot from said lab from 2008, converses with a group of people. ROBISUKE is able to even answer relatively hard, ambiguous questions.
Here’s a video (subbed in English) showing ROBISUKE talking to a single human in 2006:
http://blip.tv/play/hMQ5gouZNQA%2Em4v
And finally, the little guy you see in the video below is called SCHEMA (first shown in 2009).
SCHEMA is able to engage in conversations with up to 3 people simultaneously, which is quite amazing (even though the Japanese/English video embedded below looks a bit staged).