I used to use NetNewsWire Lite, but never really liked it, then I used the built-in RSS reader in Safari once that came along, but when I switched (back) to Firefox I started using Vienna. Changed to
Sage, a Firefox add-on, which I even got working in Bon Echo (aka Firefox 2 alpha) but when I started using Minefield (aka Firefox 3 very early alpha) Sage stopped working. Considered going back to Vienna, but then I found
Google Reader, and that seems good enough (for now).
Edit: Considering the clarification two posts down, this post is completely useless for the OP, but could be useful if anybody's looking for an
RSS news feed reader... I don't really use newsgroups, so
Google Groups is probably good enough for me...