Janus Notes 2 can be downloaded for free from the Apple's App Stores.
The applications are free to use on any platform, with a aully optional In-App purchase that will stop ads on iPhone and iPad and will stop the Mac version from bothering you.
Macintosh:
iPhone/iPad:
Just to be clear. Ads on iPhone/iPad come from Apple's iAd. The registration reminders on Macintosh are rare and not intrusive.