From the Search Engine Optimization FAQ:
Sites with .com rank higher then with <TLD here>
This is another common myth that is untrue. The only time a domain extension can affect your ranking is if the search is based by country. The country-specific TLDs (e.g. .co.uk) will have priority over non-country specific TLDs (e.g. .com or .net).
One observation many make is that .coms tend to rank higher then other domain extensions. They assume it is because .coms are given preferential treatment. This is a poor assumption. .coms seem to rank higher then other extensions because they are by for more popular then any other domain extension (there are more .coms than .net, .org, .biz, .edu, .gov, and .info combined) so they naturally have a greater chance of ranking higher vs other domain extensions through sheer quantity alone. .coms also tend to be older sites so they have had a chance to establish themselves whereas newer domain extensions have not. They have also used this time to acquire more backlinks which is an important factor in search engine algorithms.
It is also commonly believed that .gov and .edu sites are given preferential treatment from search engines. This is also untrue. Web pages on .edu and .gov domains tend to rank well because they contain quality content and many webmasters will link to their content as a result. Both of these are key elements in SEO. But the fact that they are .edu or .gov domains does not benefit them directly in the SERPs.