Mainstream Sites Serve as Portals to Hate
Yahoo and other popular Internet companies face growing international pressure to exclude racist clubs and auction items that are proliferating online; Simon Wiesenthal Center says Internet now harbors over 2,000 groups promoting anti-Semitism or white supremacy, at least twice as many as center found in spring 1999; auction sites like eBay have been targets of protest for allowing sale of items like Nazi flags; some sites have been asked to reevaluate way their search engines retrieve results; many Web services bar hate-fostering speech, but refuse to monitor or actively seek out violators; photos (M)