I like using
Yahoo SiteExplorer to check competitors backlinks. Together with the following firefox extensions you can check the anchor text used and the pagerank of the page, in Google webmaster Tools and Yahoo Site Explorer.
SEO For FireFox (similar to
SEO Quake)
SEO Link analysis
Besides these I also use the
RankChecker extension to monitor my rankings. To check your backlinks you can also use the
Harvester Tool which filters unique linking domains
