While the heyday of certain *ahem* pricing discrepancies between third party sites and BA.com are long gone (I'm thinking AA.com Intra-Europe and LAN.com to anywhere), there can still be distinct advantages of booking through third parties.

I'm thinking of:

a) Use of a different GDS leading to favourable discount fare bucket availability
b) Different sales city leading to favourable discount fare bucket availability
c) Booking agents in your own country that allow payment in your local currency thus avoiding FX fees.
d) Booking websites that allow the use of vouchers for payment (bought at a discount to face value, awarded through employee recognition schemes etc.)
e) Booking websites that generate a higher cashback (either through Quidco/TCB or through credit cards) for flight bookings than BA.com
f) Sites that allow the use of discount codes for flight bookings (rare but last minute.com we're offering a flat 5% last year)
g) Other special features such as Amex companion fares and STA Travel Youth/Student fares
h) More advanced functionality such as more flexible access to IT fares, favourable multi city functionality

Often such savings are slim and/or get offset by service/booking fees so the structure for posts I'd propose is as follows:

Line 1 (for collation in the Wiki): Booking Agent / Website | Domicile | GDS| Booking Type (OW/RT) | Distance (LH/SH) | Service Fee* | Special Feature
Line 2 Route | Fare Charged (optional) | Saving vs. BA.com

* Service fee calculated as difference between total amount charged and equivalent pricing on ITA in charged currency (if it can be verified that ITA is pricing on the same fare basis).

A final consideration is that TA fares often offer less flexibility than those booked directly (e.g. POUG, UUA) and many of you hold BA credit cards which increase the attractiveness of booking direct. With that in mind, is there any interest in establishing a master thread along these lines (or with modifications based on feedback received)? I recently saved 15% on a short haul flight applying this technique but the savings weren't worth the time spent researching which is what has promoted me to suggest a one-stop post on the subject. If there is a strong preference to keep this under wraps to avoid killing the remaining golden geese then I will go with the majority view!