New version of Art Plus Wallpaper Calendar is setting the standards for this type of programs. Not only can you display calendars on your desktop but you can also display short notes and even diary entries. You can set reminders, use built in diary or link Wallpaper Calendar with EasyNoter
(even LITE freeware version) and display EasyNoter Diary entries on your desktop.
Both LITE (Freeware) and PRO versions of the program are released. While LITE, freeware version contains less layouts and some other options are not as developed as in PRO, it is still a very nice product that you can use as long as you like.