3DS homebrew software for reading and writing Amiibo. If you have a hacked 3DS and not NFC capable phone or Arduino this is really handy!
I had a bit of trouble getting writing to work, so here're some tips:
You can keep trying to write to the same tag as it doesn't lock it till the end of the process.
Don't put the tag near the screen until Thenaya prompts you for it, and as soon as it's close enough to start writing leave it at that position. Seems to not work if it's closer than about a CM.
Sometimes I seem to need to restart Thenaya between writing tags.