Ask Jeeves is a search engine which accepts questions and returns results from searching the web. It is NOT really "Questions and Answers".
Google Answer is a user-pay service where you can ask a question and nominate a price for the answer. " More than 500 carefully screened researchers are ready to answer your question for as little as $2.50 -- usually within 24 hours." All the previously answered questions are available and forms a variable source of information.
Yahoo Answers currently in beta is a social networking based system. It is free to ask a question and anyone can answer (provided you log in using the YahooID).
I find Google answers has provided me with some answer with very in depth research whereas the Yahoo answers seems to be more suitable for student's work. This may be the result of pay verses free.