I always thought tags' potential for increasing discovery of new things is severely under-utilised.
The WordPress support forum uses tags to great effect, though there's certainly room for improvement. See this post for the Contact Form 7 plugin for instance.
Thanks to its (hidden) contact-form-7 tag, it spices up the topic with two key additions:
- A prominent plugin title
- A widget with key links:
- Plugin page
- Frequently Asked Questions
- Support Threads
Another example I like is news on IndieDB. Scroll down and you'll see there are boxes for Related Games/Engines/Groups. Tags would be good for this sort of thing.