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
• Reviews

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.

Another thing about tags is that I feel they should either denote something uniquely identifying about the thread they're applied to, or not be used at all. Take this one for instance:

With one exception, all of these tags are better covered by a search. Say you're looking for a plugin with some kind of YouTube functionality. Sure you can get a decent overview with the youtube tag, but you won't trust that to be an exhaustive list, because you can't trust people to have tagged their threads appropriately (a common issue for WordPress plugins for instance). So you search.

As soon as you go past 1-2 tags it's also very noisy (especially in combination with the Topic Badges, which I'll make a separate comment on)

The only tag I find worthwhile is imgshow, and that's assuming it would be unique to this plugin. This is where hierarchical tags (think Google Code issue tracker) can be very useful, because you could make rules for special tag groups, e.g.:

If I create the first instance of the plugin:imgshow tag, I am now the owner of this tag. I will get a notification of all new threads using this tag, and I will have moderation rights over every thread that uses this tag.

@Giorgio-Chiodi said:

There's no correct way to use them.

That's where I think forums ought to be more opinionated. With some exceptions in special categories, I wouldn't want to allow more than 2-3 tags per thread.