Gargron

This widget shows all tags that are anime titles as a dropdown menu. It creates a taxonomy called Anime, you only need to tag your posts not in the ‘Post Tags’ box, but the ‘Anime’ box.

Anime Dropdown Widget

This widget shows all tags that are anime titles as a dropdown menu. Using WordPress' custom taxonomies.

This Related Posts Plugin for WordPress displays links to a given number of Posts that are related to the current post(s) by Tags or Categories. It can Automatically add links to Related Posts at the bottom of every Post or you can drop the Widget on your sidebar.
Settings: Excluding posts, categories, and tags; section heading; number of links; display post date; and excerpts.

EZ Related Posts Footer Links and Widget

Display a linked list of related Posts by Tags or Categories at the bottom of every post or on the sidebar. Options to show Post Date and Excerpts.

Anime Dropdown Widget vs EZ Related Posts Footer Links and Widget