Effective research can be done in different levels. Use a broad search engine to identify basic arguments, then use more specific sites to find in-depth information.
A basic search engine will return most results from news sites, like the following. These are good places to start your research.
Wikipedia! (What list of web resource links would be complete without it?)
Debatepedia (Yes, another wiki)
The following are more specialized sites that can be used for more specific research on a specific subject.