It's sort of shocking that WP gives you all sorts of facilities for creating custom post types, and almost none for displaying them in the front end. I had created a CPT because I wanted a "page-like" object that also had excerpts, and could be categorized and tagged. I had created a fair amount of content with my new object (plus other content in conventional posts). I just assumed I'd be able to display archives which mixed the different post types. Wrong!!!
That's where this little plugin comes in. It's very simple: a few check boxes and you tell WP which post types you want a tag archive (or category, or whatever) to contain. Check the right boxes and you can mix all the standard and CPTs in the same archive listing.
Marvelous!!! The only alternatives I could find were included in the "Pro" versions of various CPT software systems.
Really, this deserves to be much more popular.