For top freeware addons, I would say (if you can run them adequately) FreeSkyProject (what used to be Posky), Project Airbus (although many of theirs are built for FS9), and there is a good MD-83 with a built in FMC called FSND (on simviation) amongst other good choices for planes. For scenery, I honestly just search or browse avsim and simviation to find scenery that I am looking for and much more frequently than not they have what I want and I just install it through directions on the readme. For other utilities, I would highly recommend ASE (AFCAD editor), vroute (works for FS9 I think though, so it wouldn't be new for you necessarily), and vasFMC (same FS9 story) to name a few as I can't think of any more off the top of my head.
Hope this helps,
Regards