Taxonomy breadcrumbs (using Crumbs)

Crumbs can be used to create taxonomy-based breadcrumbs.
The plugins for that are already built-in.
You can enable them on Admin > Structure > Crumbs (admin/structure/crumbs).

Prerequisites:

• Taxonomy module enabled on your site.
• Some vocabularies existing on your site.
  For the examples below, we assume having a vocabulary named "animal_categories" and another vocabulary named "tags".
• Some node types with "term reference" fields.
  For the examples below, we assume having a field named "field_animal_category" (on "animal" node type), and another field named "field_tags" (on "article" node type).
• We assume we have a views page at "animals" which gives an overview of the available animal categories.

Desired breadcrumb:
Home » Animals » Mammals » Whales & Dolphins » Flipper

Quick summary

(crumbs-7.x-2.x only)
Visit Admin > Structure > Crumbs > Entity parent > Taxonomy
Specify "animals" as the parent path pattern for "Animal category" taxonomy terms.

Visit Admin > Structure > Crumbs
Make sure the following plugin keys are enabled:
taxonomy.termReference.field_animal_category.node.* (medium priority)
taxonomy.termParent.categories (medium priority) (*)
crumbs.entityParent.taxonomy_term.categories (low priority) (**)

(*) probably already implicitly enabled via * wildcard, so you don't really need to take action.
(**) probably already implicitly enabled via * wildcard, but you need to adjust the priority.

More detailed explanations for each step:

On node pages (node/%)

We want that nodes which have a category set in "field_animal_category" use the category as the breadcrumb parent.
On the other hand, we don't want that field_tags plays any role in the breadcrumbs.

Long story short (spoiler):
Enable the plugin key taxonomy.termReference.field_animal_category.node.*, by moving it into the "enabled" section (set the priority as you see fit).

Longer explanation:

Watch out for plugin keys that begin with taxonomy.termReference.. E.g.:

taxonomy.termReference.* (disabled by default)
taxonomy.termReference.field_tags.*
taxonomy.termReference.field_tags.node.*
taxonomy.termReference.field_tags.node.article
taxonomy.termReference.field_animal_category.*
taxonomy.termReference.field_animal_category.node.*
taxonomy.termReference.field_animal_category.node.animal

The taxonomy.termReference.* is "disabled by default", which causes all the child plugin keys to be disabled as well.
This is necessary because usually only some term reference fields are supposed to have a meaning in the breadcrumb.

We now move the taxonomy.termReference.field_animal_category.node.* into the "enabled" section.
This way, all nodes that have the field_animal_category set will use that for the breadcrumb parent, no matter which node type.

Still doesn't work?
Look in Admin > Structure > Crumbs > Debug, type a node url, and see the different parent-finding plugins in action.
Maybe you need to readjust the priorities.

On taxonomy term pages (taxonomy/term/%, termParent)

You want that taxonomy term pages use the parent term as the breadcrumb parent.

Solution:
It probably already works out of the box!
The plugin key to enable would be either taxonomy.termParent.* or taxonomy.termParent.animal_categories.
However, since none of these is "disabled by default", they are probably already doing their job just fine.

Still doesn't work?
Look in Admin > Structure > Crumbs > Debug, type a node url, and see the different parent-finding plugins in action.
Maybe you need to readjust the priorities.

Top-level taxonomy term listing

We create a views page on path "animals" and want to use that as the breadcrumb parent of top-level animal categories (those that don't have another category as a parent).

crumbs-7.x-2.x branch:
Go to Admin > Structure > Crumbs > Entity parent > Taxonomy
On "Animal categories" vocabulary, type "animals" as the parent path pattern.

Then go to Admin > Structure > Crumbs to readjust the weights.
Make sure that crumbs.entityParent.taxonomy_term.animal_categories has a lower priority than taxonomy.termParent.animal_categories.

crumbs-7.x-1.x branch:
Create a custom module with a Crumbs plugin, that sets "animals" as the parent path for animal categories. Good luck with that :)
See http://drupal.org/node/1004398

You then visit Admin > Structure > Crumbs and make sure the custom plugin is enabled.