This seems to be an ignored/undocumented topic (no results from either Google or forum searches) but I thought it might help the non/mid-frequent travelers like myself or simply those who love a "golden hello". And maybe we can create a wikipost?.

Many years ago I had just registered on a frequent flyer program before my first flight on an airline (I think a Oneworld member, so maybe BA, QR or CX) and I was allowed priority boarding at the gate. On a separate occasion I remember receiving a complimentary upgrade to Premium Economy; although I believe that wasn't an "official" benefit, I think just being a registered FF member helped me get it.

Looking at a few airline websites that I fly, here are a few examples I found:

CX Marco Polo Green ($100 enrolment fee): Marco Polo counters, Priority Boarding, Some extra baggage allowance, Redemption benefits for lounge and extra legroom.
BA Executive Club Blue: Priority waitlist, Member-only offers for magazine subscriptions and special redemption deals
SG KrisFlyer: Boarding Pass Privileges all year round, which include "Exclusive discounts on hotel stays, car rentals, restaurants, boutiques, shopping malls and other tourist attractions".

What have you found, and have you also experienced any "unofficial" benefits?