Tom's Coordinate Converter.
Convert GB Eastings and Northings to WGS84 Latitude and Longitudes.
I got so tired of converting eastings and northings to latitudes and longitudes that I wrote my first ever desktop Windows application to help. Paste comma-seperated Eastings and Northings in at the left, get comma-seperated Latitude and Longitudes out on the right. It's that simple.
Download coordinate converter here, unzip the folder, and run the *.exe.
Built on John Newcombe's GeoUK Library for .Net.
Download the full source-code as a VS2015 solution — also under LGPL license for now, but happy to be as permissive as you like if you email me.