I started using this so that I could use the D3 RMAH and I did not have an Android / iPhone (I use a Symbian Nokia ) Works quite well I, admit.

And no, I did not make it.

If you want to play safe, just get an official authenticator. This is for us cheapskates.

Hosted here.
http://code.google.com/p/winbma/

Alternative with GW 2 support (I use this one myself).
http://code.google.com/p/winauth/

THIS AUTHENTICATOR IS NOT SUPPORTED BY BLIZZARD AND IS BASED UPON EMULATING THE ANDROID VERSION OF THE AUTHENTICATOR ON A WINDOWS PLATFORM.



Info copied from the site.
Spoiler


Quite nifty I must say.