Tagging is an upcoming feature that is being developed right now.
It will be released in two phases.
The first is item classification, where the contributor of content (or anyone with write access) can add tags to label their work.  Community members can then find other content that has that label.
The second is social tagging, where any community member can add his or her own tags to any content.

Update:  Labels was released September 25th and we are now working on social tagging.

Do you have any feedback on Labels?

Love it, and love the flexibility. We're using to organize our posts and documents. it would be nice to be able to label pages as well. It would also be nice to have some sort of way to create a "tag cloud" of sorts to display labels (in a widget for example) so people can click on labels as an easy way to search labelled items. Looking forward to social tagging.

The ability to label pages will be here by end of this quarter (Dec 2009) - along with some other significant improvements!  Make sure you are subscribed to the Product News blog channel for announcements coming out in the next week.

I'll pass on the "tag cloud" widget idea to our development team.

Thanks for the great feedback!