Canonical URLs: an SEO's best friend if used correctly. But also a source of major SEO fails if used incorrectly.
It can be hard to keep track of all the canonical cross-references. That's why I've added the Canonical Report to SiteGuru. It shows you which canonical URLs are used where. To make crawling as efficient as possible, it also highlights which canonicals are indirect (i.e., a canonical which has a canonical itself).
The canonical report helps you check if your canonicals are correct. A great way to avoid an SEO disaster in the making.
The Canonical Report is available for all SiteGuru users. You'll find it in the Site Report in the Technical section.
Refresher: what are canonical URLs?
Canonical URLs are a great way to prevent duplicate content. If you're using parameters in your URL, for filtering or sorting, you don't want all these variants to be indexed. By adding the URL without those parameters as the canonical URL, you prevent duplicate content.