At the highest level, PulseWire provides areas for posting such as Resource ExChange, Sharing Solutions, etc. Within those sections, however, we've taken a tag-based approached to post categorization. In recent years tagging has become a more popular method of categorization due to its user-driven nature (see, for example, YouTube and Flickr). However, in some cases a more top-down approach has remained the norm (such as Craigslist). We chose to go with tags, because we didn't want to force content into pre-determined classifications; rather, we're allowing and encouraging PulseWire's users to drive the direction of the content and, through tags, show us what topics and categories are popular.
While this is working for now, as the site grows it may become necessary to modify this approach. The Resource ExChange, for example, may become too difficult to navigate via only tags when there are thousands of posts every day. At that point, a hybrid approach could be useful, with predefined categories available for browsing but with tag browsing still available. Each predefined category could cover a set of tags and be created by looking at which tags are most popular.
This would be one option, but it isn't the only one. We welcome input and feedback on what is and isn't working as methods for content classification and browsing. If you have any ideas or example web sites that have implemented a useful, usable, and user-driven classification system, please share!
Related reading: Why we need librarians