In regard to tags and categories, version 4.0 appears to support those concepts natively.

For pre-4.0, however, you can still do this, just with a little more work. I haven't implemented Categories on my site, but here's how I did tags:

First I added a new text field to the blog post document type, called BlogPostTags. Then for each post I enter a comma separated list of tags in this field (optional).

I then wrote a static method in my utility library (App_Code\<SiteName>\Util.cs) to accept a comma separated list and return a list of links (with the specified tag as a "tag" querystring parameter) to the "find by tag" page that I created.

In any transformation, I can use a bit of code like this:

<%# Util.GetTagLinks(Eval("BlogPostTags")) %>

Then on the "find by tag" page, I placed a repeater that has a "where" clause that says something like this: BlogPostTags LIKE '%{%tag%}%'

You can see this in action here:
http://www.elijahtaylor.com/Blog/Find-By-Tag.aspx?tag=seth

Hope this helps!
Elijah