How to list categories dynamically
It sounds like a View of Taxonomy terms would meet your needs.

If I wanted to create a block with a list of items tagged with term in the category "Electronics" then I would do something like this:


Create a View with the following options:

Show Taxonomy Terms
of type Electronics
sorted by [unsorted]

Check the "created a block" box.
Add terms "DVD player", "DSLR camera", and "LCD monitor" to my "Electronics" vocabulary.
Add the relevant terms to the relevant product pages
Enable the block


Now, if I start selling Zunes next month (hah) then I can add term "mp3 player" to the "electronics" vocabulary and it will show up in the list of electronics-related terms in my block.