This little gem converts from the Bristish Ordnance Survey's own easting/northing OSGB-36 coordinates to the WGS-84 latitude/longitude we have come to know and love (and is used thoroughly in GPS, Google Maps and all that stuff).
require 'ordnance_survey_vs_the_world'
easting = 531182
northing = 182409
gr = OrdnanceSurveyVsTheWorld::OSGridRef.new(easting, northing)
ll = gr.to_latlon
ll.lat # => 51.5248579430871
ll.lon # => -0.108828758467826
Was feeling like it.
Public Domain for yeah!