Counties of Great Britain
Thanks for using the program. Please do comment and let me know what you think...

Where does the map data come from? The basic data is available free of charge from the Ordnance Survey, the UK government's mapping agency. ("Ordnance" means weaponry: the agency started in order to work out how to haul cannons around.) I used the GIMP to colour each county differently, and then used boutell's excellent GD library to turn the result into an image which used a separate palette entry for each county. Finally, a small Python script can serve up the correct images quickly because all it needs to do is rewrite the image's palette.

Where's Northern Ireland gone? No idea. It was missing from the Ordnance Survey's mapping data; I'd have put it in otherwise. You should ask them to include it. ias has said in a comment below that NI has its own mapping agency, which is why it doesn't appear on OS maps. (The NI version of OS don't appear to give any maps out for free, either.)

You've got the counties wrong... Possibly. For those who don't know, the UK's counties are nothing like as stable as, say, American states; new ones spring up every few years, old ones vanish, the boundaries shift, and people, even the government, keep using abolished counties anyway for some purposes. I've mostly used the current counties, though slightly less so in England; in an attempt to reduce confusion, I haven't shown county boroughs separately from counties (so, for example, Nottingham is shown as part of Nottinghamshire), I've joined some neighbouring metropolitan counties together, and occasionally used names as they were a decade ago where I thought it would be simpler. I've also given up on any attempt to partition Yorkshire correctly. If you think any of this is wrong, let me know.

How about a map of European Union members / German Länder / Australian states / Texan counties... It shouldn't be a problem to adapt the code, if you can show me where to get the map data for free and you're willing to do some sanity-checking of the results. Let me know.

How do the URLs work? Briefly, they contain an encoded bitstring. Each county has its own bit which is set if it's yellow and clear if it's blue. The bits are encoded into a base-64 string (not the same base-64 encoding as in MIME). You can find out more from looking at the JavaScript in the tickybox page; let me know if you want more information. The source is available if anyone wants it.

Thanks go to: firinel, naltrexone, haggis and riordon were the first testers. naltrexone did a wonderful job of making sure the JavaScript worked on Internet Explorer as well as Mozilla.