How to find relevant and new open research topics?

Try to find papers giving a state of the art review on your field. They'll be easy to find, they generally have a lot of citations and written by the experts in the field. Some of those review papers (ones I've read at least) provide suggestions to researchers as to what needs improvement in the field. Good luck.

Here is a good article on choosing the research topic aimed specifically at the grad students:

http://chronicle.com/article/Choosing-a-Research-Topic/45641

Some further advice of more general nature (and partly applicable only post Ph.D.) is gathered here:

http://aclinks.wordpress.com/2009/05/09/how-to-choose-a-research-topic/