OpenStreetMap
The UK post office's response to a Freedom of Information Request (http://www.whatdotheyknow.com/request/location_of_every_post_box_that) asking for the location of post boxes was somewhat less helpful than had been hoped - they did provide a list but the locations were rather approximate.
So the obvious next step was to crowd source the job of working out where they really were, which somebody has now done, using OpenStreetMap maps of course.
So if you want to help pin match the official list of post boxes to the ones you've found while mapping, here's where you can do it:
http://www.dracos.co.uk/play/locating-postboxes/
Discussion
Comment from njd27 on 6 November 2008 at 11:23
It would be nice if you could search for more of the partial postcode: for instance there are postboxes listed in "SO16 7" but if I put "SO16 7" in the search box no matches are found.
Comment from Mappo on 6 November 2008 at 14:22
It appears the provided list includes postboxes contained within Post Offices, but the OpenStreetMap data on Post Offices isn't shown by balloons (but you can see a circled envelope on the map at appropriate zoom levels). For example, PA1 1AA – Paisley Post Office | 34-36 Central Way (ref. PA1 2) which I have just matched manually.
Comment from TomH on 6 November 2008 at 14:55
Just to make things clear - that isn't my site, it's Matthew Somerville's, so if you have suggestions for improving it you should aim them at him.
Comment from eAi on 7 November 2008 at 02:49
I've done a few I know of.