Tags are a great way to harness the power of Highrise and help you drill down to a specific group.
For example, we use "via" tags to keep track of how we originally met contacts. So there's a "Via Marc Hedlund" tag on everyone introduced to us by Marc Hedlund. That lets us quickly determine how the introduction happened, which can come in handy when someone calls out of the blue.
We also use industry tags to group people in similar buckets. For example, we've tagged everyone we know in the press as "Press". Now we can just click the Press tag and see all the journalists we know — without having to know their exact name, company, title, or affiliation.
Other tag ideas: Health care, legal, vendors, customers, etc. If you have an interesting way to use tags, let us know.